This also adds some comments around ModModel.cpp and ModPage.cpp to add
some ease of reading the code.
Also move some things from headers to cpp files.
This is done so that 1. ModAPI behaves more like an actual API instead
of just a helper, and 2. Allows for more easily creating other mod
providers that may or may not use network tasks (foreshadowing lol)
Moves all things related to creating the URLs of the mod platforms
that go to network tasks to a single place, so that:
1. Maintaining and fixing eventual issues is more straightforward.
2. Makes it possible to factor out more common code between the
different modplatform pages
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.
