Do not manually hide categories that have no results in Fedora

Other distros will have a different set of packages.
Add gs_category_get_size() to get the number of applications in each category
Do not show empty categories in the overview screen
Do not show subcategories in the categories screen with a small number of apps

The number chosen here is 4, but that's personal preference.
Apps not shown are available in the 'General' subcategory.
Count the number of applications when getting the category lists