This creates a hierarchy in which ModPage and ModModel are the parents
of every mod provider, providing the basic functionality common to all
of them.
It also imposes a unique .ui file (they were already equal before, just
duplicated basically) on all mod providers.
Fixes#206 partially. Although we don't list mods that have no
compatibility with the mod loader we are using, mods that have support
for both loaders still show up, and the versions for both the loaders
are still shown.
Also simplifies a little the logic in
FlameModIndex::loadIndexedPackVersions
