SVGHMI: fix some JS + add generation of detachable elements, to be continued

SVGHMI: now compute discardable elements first so that we can do better grouping of required elements when sumarizing.

SVGHMI : documentation for new geometry intersection computation code + little fixes...

SVGHMI: detachable and discardable elements sets, Reworked geometric intersection, toward more accurate page content detection.

Moved page's widget/element dependency crawling functions so that it is possible to compute a global detachable and discardable elements sets.
Reworked geometric intersection detection logic to distinguish ovelapping and inclusion.
Goal is to include englobing and overlapping graphical elements, but not groups (would then include everything around...). Intermediate commit, to be continued.

SVGHMI : still trying to optimize. Added xslt code to identitfy minimum set of elements needed by a particular page. Plan is to remove unseen/unused elements from the DOM, and re-appending them later when used, on page switch. Disabled previous optimization.

SVGHMI: attempt to optimize for webkit, because of really slow style recompute in svg:use : "unlink clones" (as in inkscape) but stop deep-copy when meeting a widget, and create a new clone instead.

SVGHMI: optimize browser work by hiding (display:none) pages that are not displayed.

SVGHMI: Better and less verbose SVG namespace handling in XSLT transform

SVGHMI: Added mostly untested switch widget

SVGHMI: HMI:Page can now be a svg:g group or a svg:use clone, and included/linked widget are included in page. HMI:Jump was updated to be clickable through svg:use, to be generalized.

SVGHMI: ensure that coordinate in CSV file generated by inkscape are both matching svg default unit and in default reference frame

SVGHMI : still behave, even if important things are missing in SVG widget definitions (ex: needle missing for Meter widget)

SVGHMI : fix watchdog exception when timeout null

SVGHMI: be a bit more tolerant with missing HMI paths or missing elements in widgets : continue build (with warning) and fail silently at runtime.

SVGHMI : fixed bug happening when some SVG ids are containing dashes

SVGHMI : wait for initial timeout after watchdog is triggered, since it generaly induces HMI restart

SVGHMI : watchdog is now taking an initial and interval duration as CTN fields.

SVGHMI : avoid caching of xhtml file by forcing cache-control header in http response.

SVGHMI : HMI_STRING now also supported from HMI to PLC

SVGHMI : cosmetic

SVGHMI non significant fixes

SVGHMI : support for HMI_STRING and HMI_BOOL

plc_debug.c/var_acces.c : whitespace cleanup and other cosmetic changes

SVGHMI : Added python fomating {port} and {name} to commands so that command can build target URL

SVGHMI : add on Start, Stop and Watchdog command fields to configuration

SVGHMI: added a watchdog. To ensure that the whole chain is checked, watchdog use a periodic echo of a hearteat variable. JS client code systematically register /HEARTBEAT at 1s update freq, and reacts on updates of /HEARTBEAT by systematically incrementing it. C code catch /HEARTBEAT update and feeds python-implemented watchdog. For now, watchdog does nothing when tiggered

Add --on-plc-start --on-plc-stop and --status-change to runtime command line, calling given command respectively on start stop or any event. Command line string is python formated (to eventually include status with {}) before being split (supports quoted strings) and passed to Popen for non-blocking execution.

SVGHMI : had to move the problem of wkaing up python thread from plc thread to platform specific code.
Since Xenomai's cobalt thread are definitely incompatible with normal posix python interpreter binary's thread, we must synchronize them with arcane rt_pipes (the only ones that really work cross domain) as already done in debug and python async eval blocks.

SVGHMI: change collect/send thread looping condition to fix infinite loop in some cases

SVGHMI: re-implemented tree view with classic wxTreeCtl

Non significant changes, whitespaces, etc.

SVGHMI: Added simple HMI Tree View.

SVGHMI: add a class attribute to HMI Tree nodes, set when using HMI_NODE

SVGHMI: HMI_LABEL and HMI_CLASS become HMI_NODE.
- Name of parent POU becomes HMI tree node name,
- Name of HMI_NODE variable becomes class of the node.

Add FatalError() method to ConfigTreeNode.
Extensions can report error addressed to user, without a traceback.

SVGHMI: various fixes to make SVGHMI behave on more versions of twisted and GCC.

SVGHMI - prepare page with cached data when switching. This prevents values that do not change and that was already subscribed in previous page from keeping undefined.

SVGHMI: various insignificant code moves, commenting and typos fixes.

SVGHMI : fancier second page in tests/svghmi

SVGHMI: SVG viewport now defined so that HMI take scales and fit to the view. Implemented page switch through viewport change, no hiding of widget for now.

SVGHMI - added simple Meter widget.

SVGHMI: Quicker update path for input widget when pressing on buttons, do not wait until data comes back, and simply update value text of the pressed widget. Updated PLC prog for more amimated value to display

SVGHMI: fixed HMI->PLC dataflow : not updates as expected, and not initialized properly after subscribe.

Fixed one more sequel of 5f79b194fa63 'SVGHMI: filter out temporary variables created while generating ST code out of FBD.'

SVGHMI: Added relative changes of HMI value from widgets.

SVGHMI - Fixed svghmi.{c,js} about HMI -> PLC data unpack.

SVGHMI: Added init call to all widgets at startup to bind events. More features in Input widget : Edit and Change buttons. WIP HMI->PLC value update, incoherent data detected in C part on update.

SVGHMI: dispatching data to minimalist "Display" text widget.