Add support to delete reuse

[Matthias Schoettle]

Currently, when a reuse (in form of an instantiation) inside an aspect is deleted, the reuse in the feature model is not removed.

When removing the reuse, everything (instantiation and reuse in the feature model) should be removed.

Issue #271 is related to this.

As discussed in our meeting on 05.03.2015, the reuse should only be allows to be deleted if there are no COREModelReuses left. In order to know this, there needs to be an opposite reference (currently modelReuse, but should be renamed to reuse) named modelReuses.

[Matthias Schoettle]

Also, when removing the last reuse of one concern, we might want to remove the imported concern as well ("unimport").

[Matthias Schoettle]

Finalized by Cécile. The reuse can only be deleted if there are no model reuses left (using the X gesture). In the future, we might want to add the tap-and-hold option as well (using the menu).

[Matthias Schoettle]

Deleting a reuse should also remove the imported concern folder if there is no other reuse for that concern.