Fix up an oversight

There was a leftover reference to the old name of the box
on the overview page. Fix that.
Rename General to Other

This is the 'leftover' subcategory, and 'Other' is a more
fitting name for it.

https://bugzilla.gnome.org/show_bug.cgi?id=707550
Sort 'Other' to the end

That seems fitting, since it is 'everything else'.
Move the hardcoded menu spec plugin earlier

We move this to the beginning of the list, so that other
plugins can add subcategories to the main categories that
this plugin creates.
Add hardcoded 'featured' subcategories

Since we don't have popularity data yet, we add a
hardcoded 'featured' subcategory to each category,
and populate it with some hand-picked samples.

The samples were chosen by Jon Mccann.
Sort the featured subcategories up front

Moving the 'featured' subcategories to the first position
automatically makes them selected initially, which is the
intended effect.

https://bugzilla.gnome.org/show_bug.cgi?id=708295