Use a QPointer for CategoryManager's model.
Use a QPointer for CategoryManager's model.

We don't take ownership of the model used in the CategoryManager. Said
model can be deleted any time, and setModel called several times (if a
second new ModelStack instance is created). Reflect this relationship
properly with a QPointer instead of a plain pointer.

Caught using the first test case.

sandsmark/zanshin,sandsmark/zanshin,kolab-groupware/zanshin,kolab-groupware/zanshin,kolab-groupware/zanshin,sandsmark/zanshin