Refactorizes FragmentActivities and MainActivities [end]

Refactoring concerns:
 * HomeActivity
 * SoloPlaylistActivity
 * GroupsMainActivity
 * SoloMainActivity
 * UnisonMainActivity
 * UnisonFragmentActivity

Organizes the import for the whole project (that's why so many files are updated,
but for most of them only the imports are changed).