SVGHMI: added dropdown selection highlighting and fixed scrolling so that it doesn't miss any entry while jumping from one page to the other.

SVGHMI: change scroll buttons into single unicode triangle and center them. Use highlight rectangle duplicated and hidden to catch selection clicks so that the whole row is clickable.

SVGHMI: DropDown widget now using new class based style

Add a little test to see OnChange's .count .first .last in action

Fixed "Onchange" object added in previous commit

Python Safe Globals now have more reliable triggering of OnChange call. Added "Onchange" object to accessible runtime variables that let user python code see count of changes and first and last values.

SVGHMI: cosmetic fixes in button widget

SVGHMI: Rewrote button widget.

SVGHMI: update generated XSLT

SVGHMI: stuff test so that it moves all the time, to have better chance to spot memory leaks...

SVGHMI: prevent unwnted context menu and pinch zoom

SVGHMI: more attempts to prevent losing memory in JS closure

SVGHMI: WIP trying to reduce memory usage : use .onclick onstead of SetAttribute, avoid useless closure and object creation when possible, etc.

SVGHMI: update generated xslt file

SVGHMI: use const when applicable