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 
It should work correctly, the app manager doesn't cache any result derived from what users the group is in (unlike `OC_App`.

As long as the group manager returns correct result the results from the app manager will be correct, even if the logged in user changes, the groups change etc.