Make activity types management (component assigning) more consistent and less CiviCase specific

As a consequence of CRM-3864, activity type management has exceptional case for CiviCase and is (in my opinion), a bit non-consistent. While it's pragmatic and totally fine to keep it this way for 2.2, it would be good not to forget about potential improvements that can be done in this part of the code and spend a bit of time to think/discuss about it when good moment comes.

Potential rework might also affect component infrastructure heavily (components registering their own activity types).