Improve list of available languages for UI l10n.

Improve list of available languages for UI l10n.

Add a new tag HasGuiSupport to language file. Add it for all l10ns
that we currently ship. The po files that are unused are not currently
tagged as available, but this could be done, since the code later
checks that the translation is actually there.

This new information is used in GuiPrefs when populating the language
combox.

The new scheme implies that adding a new language is now a two-step
process:
* the language code has to be added to po/LINGUAS, as before;
* one of the entries of the lib/language file has to be selected as
  reference and be given the "HasGuiSupport true" property.