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 
Are you sure ? I remember debugging into the group manager and seeing it cache groups.
But I guess that the group manager will become a different instance after the "session/LDAP" apps are enabled