Documentation-wise in C, this doesn't matter a lot, but it allows
GObject-Introspection based bindings to use their built-in versions when
they want to render any kind of documentation (for example, docs for
Python plugins can render `%NULL` as `None`).
We were doing it all the wrong way, fixing one combo box object at a
time. So this commit basically reverses commits 68a33ab5bd, 6dfca83c2a
and a9a979b2d0 and instead runs the same code in the class code. This
way, all objects based on these base classes will have the fix from
scratch.
These improved various other drop-down lists (I found some of them, and
probably not all) as I fixed all GIMP custom widgets based on
GtkComboBox.
Note that it has to be run after filling the list apparently (I had the
problem especially with GimpIntComboBox if running in the _init() code,
then the list widget showed wrong).
Better factorize by reusing code rather than recreating a combo box
which does basically the same thing. I only added a boolean parameter to
retrieve only the sublist of manual language.
It also takes advantage of the self-translated language names from
initialization.
Derive GimpTranslationStore from GimpLanguageStore. It lists the
installed GIMP translations. Also sort the list alphabetically
instead of relying on the XML file being sorted.
