SVGHMI: removed dead code

SVGHMI: Add "lang" permament persistent HMI_LOCAL variable to reflect selected language, apply stored language choice at startup and make it always subscribed to a pseudo widget (as for hearbeat) that apply language choice when it changes.

SVGHMI: Added HMI:VarInitPersistent to initialize persistent HMI_LOCAL and PAGE_LOCAL variables, stored as cookies in browser.

SVGHMI: Enforce formating HMI_LOCAL and PAGE_LOCAL variables until the end of variable name

SVGHMI: generate JS translation literals so that it is possible to also translate uses of texts, and added JS code that apply translations to tspans

SVGHMI: widget activable elements : moved code to wher it belongs, rewrote cleaner

SVGHMI: update generated XSLT

Merged

SVGHMI: make a copy of tests/svghmi as test/svghmi_widget because too crowded since having pages for widget documentation. Strip those pages from tests/svghmi, so that it
is more light for regular testing.

SVGHMI: Prevent excluded widget to appear in generated pages' widget list.

SVGHMI: Remove recursive walk in parsed widget to find unique type and use xsl:key instead.

SVGHMI: Fix browser side exception when some widget are not used, and are then discarded and not present in final SVG. In that case JS code was still making reference to discarded widget elements and was raising exception at init.