SVGHMI: apply per class overloading of sub/unsub to ForEach widget

SVGHMI: Widget's specific sub/unsub traits are now wxpressed in widget's class.

SVGHMI: Parameters common to all widget constructors now passed as ordered parameters.

SVGHMI: One class per widget type, widget objects are instances of these classes, and members are passed through constructor. This allows to keep compatible with previous widget_defs template used in most widgets.

SVGHMI: Add an exslt function to collect used widget types, in order to only include necessary JS definitions in furure changes.

SVGHMI: Systematically output local name as /* comment */ when emiting content.

Merge

SVGHMI: Fix flawed logic to place nodes in the HMI tree, leading to wrecked tree in some cases.

Added button and circular bar widgets.

SVGHMI: added more meaningful namespaces to emit javascript code from.

SVGHMI: moved JS code of foreach and jump widget in corresponding widget_*.ysl2. To be continued.

SVGHMI: spread JS code from svghmi/scripts.ysl2 in other .ysl2 files, using dedicated preamble and epilogue namespaces

SVGHMI: Introduced "emit" to simplify producing per-topic code and content from anywhere in code. Applied to debug output, next to JS.

SVGHMI: Move reflective debug tags from "reflect" to "debug" namespace, preparing for per-purpose namespaces.

SVGHMI: Added scripts.ysl2

SVGHMI: moving code wher it belongs, and reformulate some declarations for better readability.

SVGHMI: HMI:DropDown: more documentation.

SVGHMI: Comments in HMI:DropDown source code

SVGHMI: HMI:DropDown menu aligns to selected when opening

SVGHMI: When clicking outside of opened HMI:DropDown box, close it.

SVGHMI: Fix HMI:DropDown flicker on close.

SVGHMI: HMI:DropDown now temporarily takes content from widget's arguments, until HMI:List is implemented.

SVGHMI: Connect HMI:DropDown to value in HMI Tree, in svghmi test as well.

SVGHMI: fix probable problem in HMI:Input, where value could be applied to wrong index if used in relative page.

SVGHMI: HMI:DropDown element is brought in front when opened, so that it isn't occulted by other widgets at the same level.

SVGHMI: HMI:DropDown now catches all clicks when open

SVGHMI: Add button element to HMI:DropDown, also fix problem with computed box margins.

SVGHMI: HMI:DropDown now scrolls forward and backward

SVGHMI: progress on HMI:DropDown - now selects some value on click, and close

SVGHMI: HMI:DropDown work in progress, intermediate commit. Now menu's rectangle grows up to viewport borders.

SVGHMI: intermediate commit while working on dropdown widget.

Here is the plan :
HMI:DropDown : svg:g of svg:rect + svg:text
rect is extended to match content size, and if content size exceed page size, user can scroll
HMI:List : either HMI:List:ListName as svg:text, one tspan per list entry
or HMI:List:ListName:Foreach:HMI_CLASS:SUB/PATH/TO/VALUE@/ROOT/PATH as empty svg:g

SVGHMI: HMI is not speculating on PLC variable update anymore when sending new variable value.

HMI variable cache in JS was changed in advance for more responsive behaviour when updating a HMI tree variable. This was leading to display value different from hmi tree variable in case the expected update did not happen because PLC program did revert it.

SVGHMI: finished shift and capslock support n keypad widget. Added a helper in widgets_common to collect subelements likle active/inactive/disabled...

SVGHMI: fixed keypad JS regarding shift/capslock and other details

Fix unmatched "Courier" font for monospace fonts on latest ubuntu 20.04. Selected "FreeMono" instead, present since at least 18.04

SVGHMI: Keypad working for HMI_STRING, still Shift/CapsLock not finished.

SVGHMI: fix keyboard not showing

SVGHMI: Added keypads to svghmi test's SVG file

merge

SVGHMI: Keypad: added code to re-attach keypad when starting edit.

SVGHMI: removed debug code

SVGHMI: More infrastructure for editing values with a keypad.

SVGHMI: Widget input buttons should be clickable through svg:use (onclick attribute instead of addEventListener)

SVGHMI: moved dead code to where it belongs

SVGHMI: empty widget_keypad.ysl2, WIP

SVGHMI: HMI:Switch now accepts boolean values 'true' and 'false'

SVGHMI: Jump widget, if it has a 'disabled' labeled element, reflects value of the pointed HMITree variable by showing this element when value is False, and behaving normaly otherwise.

SVGHMI: prevent man-made memory leak by limiting back button history items count to 42

SVGHMI: collect debug data through xslt reflectivity instead of yml2/python trick

SVGHMI: Jump widget can now display as active or inactive, if corresponfing "active" and "inactive labeled elements are provided.

SVGHMI: added Back widget.

SVGHMI: Jump widget now make errors if path given for relative jump doesn't match target page reference path class.

SVGHMI: Check that explicit relative page jump path is consistant with destination page reference path.

SVGHMI: ForEach Widget now has a better wrap-around when scrolling : hit the boundary systematically before wrapping

SVGHMI: Alow relative relative Jump. (i.e, relative jump from a foreach widget)

SVGHMI: Generalize applying cache to widgets, be it from page or ForEach widgets. Now ForEach widget buttons work as expected.

SVGHMI: Widget ForEach buttons now call corresponding methods.

SVGHMI: ForEach now has working (un)subscribe. Fixed PageSwitch that wasn't behaving when jumping to current page with another path.

SVGHMI: Easier way to match HMI tree elements to paths. ForEach widget now looks for paths and indexes of all items, and enforce path to be consistent with ForEach items sub widgets

SVGHMI: added func:get_hmi_tree_elt to match HMI tree node from path. Continue implementing ForEach widget : force order and completeness of items list. Now also collecting ForEach buttons.