Add \OC\App\Manager to handle enabling/disabling apps

Add a proper public interface for enabling/disabling apps.

I would like to expend this in the future and refactor all off `OC_App` in a proper public interface later.

cc @DeepDiver1975 @PVince81 
Do we need `$forceRefresh` like in `OC_App::getEnabledApps()` ?
There is a case where we first need to load the "session" apps and then refresh the enabled apps list afterwards, because the group manager requires the session.

Since this is about **installed** apps, not **enabled** apps, I guess this addition is not needed ?