Merge pull request #5 from vitw/wxPython4

update installation instructions

docs: add additional dependencies to installation instructions

update installation instructions

SVGHMI: fix svghmi_jumps example's SVG file (viewport was misplaced)

SVGHMI: rework the way cached values are applied to widgets when fresh data is missing.

SVGHMI: re-arrange Animate and data Dispath code paths.

SVGHMI: add an exemple that demonstrate use of Jump widget in different use cases.

SVGHMI: refactor an factorize widget dispatch() method calling

SVGHMI: fix ToggleButton not always reflecting current state

IDE: fix exception transfering or reconnecting after tracing variables

SVGHMI: Add predefined assignments to jump widget to notify PLC or other widgets about jump.

SVGHMI: continue rework of Jump widget to cope with Enable Expressions.

Questioning widget's activity and disability all together.
Now active/inactive/disable masquarade is all managed in widgets_common.
Propagated changes to widget already using activity (buttons)

SVGHMI: rework Jump widget to cope with Enable Expressions.

Relative jumps are not disabled implicitely anymore when pointed HMI_NODE is 0.
Also, "activable()" implementation was enhanced to make it optional.

SVGHMI: add support for "enable expressions" with arbitrary variable name assignment.

HMI tree paths can be prefixed with a variable name "@name=/MY/HMI/VAR"
Widget declarations can end with a "#" followed by a JS expression that refers to name given to variables.
Widget is disabled if expression's result is false.

Commit includes some more-or-less related generated code refactoring, that should simplify extending widget's variables attributes.

SVGHMI: preliminary implementation of a general enable bit for all widgets, using "#" at the end of widget declaration, and a "disabled" element that becomes visible instead of widget when disabled.