New xgettext method to i18n module

[bruchar1]

This method call xgettext to extract translatable string from source files into a .pot translation template.

It differs from a plain CustomTarget in three ways:

• It accepts build targets as sources, and automatically resolves source files from those build targets;
• It detects command lines that are too long, and writes, at config time, the list of source files into a text file to be consumed by the xgettext command;
• It detects dependencies between pot extraction targets, based on the dependencies between source targets.

This does the specific thing I need for my project, that I was trying to implement in a more generic way in #12272 and #11822.