This patch changes the drop named material tooltip to give feedback to
the user what is going to happen when they invoke the change.
There are 3 states:
- "": Operator will be canceled as not all data is present (dropping on background.)
- "Drop <named material> on <object name> (slot <slot number>, replacing <current material in slot>).
- "Drop <named material> on <object name> (slot <slot number).