Changed keyboard to show on defined position.

* svghmi.js
added size paramter to edit_value
changed show_modal function to offset keyboard if paramter passed

*widget_input.ysl2
in on_edit_click:
function get position if key_pos element is defined in input widget

*widget_keypad.ysl2
Reworked keypad widget to extand class widget,
added support for changing keyboard position to user defined location

Reworked button widget so it uses classes.

SVGHMI: allow multiple variables and formatting in Display widget. Formatting is printf style and given as first argument. If no formating is given as widget argument, space separated.

SVGHMI : have to move some JS declarations from "preamble" to "declarations" and others from "declarations" to "definitions" section to make room for JS libraries that would be used by widget classes.

SVGHMI: JS refactoring continued : "dispatch_value_to_widget" becomes widget class member "new_hmi_value" (was also broken by previous relativeness commit)

SVGHMI: widgets are not anymore binary relative or absolute, but have a "relativeness".

Because of allowing multiple variables per widget, we must distinguish if individual variables is relative to page, not the whole widget.

SVGHMI: More JS code refactoring : change_hmi_value and apply_hmi_value now methods of widget class.

SVGHMI: Better JS code semantic in ForEach widget class. Update lagging generated xslt.

SVGHMI: Add ForEach Widget is svghmi test

SVGHMI: add get_idx(index) to widget objects to factorize a bit code repeated accross widgets. To be continued.