Adding @defgroup and @ingroup doxygen tags into all headers
in the common folder that contain doxygen blocks.
This improves the structure, readability, and findability
of information in the resulting output.
This commit targets purely structure and does not deal with
the content of the currently existing doxygen documentation.
For this I added a convenience version of getLanguageYesNo, which works on the
currently setup GUI translation language. All other code which requires this
acan also use it instead of having to worry about the USE_TRANSLATION and thus
having two code paths.
