Ability to rename custom properties
Merge pull request #803 from Arnaud-Trouche/rename-cust-prop

Ability to rename custom properties
Fixed up Rename Property action a bit

Now it works properly for multiple objects, only changing the name of the
property and never the value. Before, the action would cause the value to
become the same for all objects.

Also, it now only creates a single entry on the undo stack.