xliff2po: Add support to convert XLIFF files with .xliff extension

Fixes #3202.
Update the check for languaje in rc files to be ok if is globally set and the actual block don't have one or is set locally with the correct one.
Added one line comments ("//") and TEXTINCLUDE secctions of Visual Studio Editor, both to be ignored.
Also, change pass to continue as if we know that is a block that must be ignored isn't needed to check other types.
Fixed that comments don't append because was using the wrong variable name.
Created a little test of rc file format that check that translatables strings are collected correctly.
Added a test for rc2po converter.