Make module categories (almost) great again

Make module categories (almost) great again

The category tag was rarely used and thus not very useful. This adds
categorization to most modules (the rest will follow) and uses the
\DeclareCategory tag we use in layouts rather than the extra syntax
we used in modules. Categories are now added to the po files and
translated.

Note that this is work in progress: the current categories are still
subject to change.

The ultimate goal of this is to sort the modules in the GUI by category
as we do with layouts, examples and templates (and add a filter to search
for specific modules)

As it is now (with the many modules we accumulated), the module selector
is not really usable anymore. If you don't happen to know how exactly a
module is named, selecting a module is really a PITA.