Use QPointer to clean up menu items after their deletion
Use QPointer to clean up menu items after their deletion

This causes the pointers to be zeroed when the objects are deleted
allowing the null check in onViewPropertyChanged to work and avoid a
segfault when closing tomviz.

OpenChemistry/tomviz,OpenChemistry/tomviz,mathturtle/tomviz,OpenChemistry/tomviz,mathturtle/tomviz,mathturtle/tomviz,OpenChemistry/tomviz,cryos/tomviz,cryos/tomviz,cryos/tomviz