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 
Should we still call it `IAppsManager` ?
I'm worried that we'll have a thousand `IManager` classes all over the place but with different namespaces... easy to be confused or trigger unwanted frustration.