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.