Blender Git Loki

UI: Improved feedback when dropping is not possible on drag 'n drop

* Allow operators to show a "disabled hint" in red text explaining why dropping
at the current location and in current context doesn't work. Should greatly
help users to understand what's the problem.
* Show a "stop" cursor when dropping isn't possible, like it's common on OSes.

Differential Revision: https://developer.blender.org/D10358