Blender Git Loki

Support passing operator to invoke on activating or dragging an item

An annoyance is that the operator names have to be passed to the template which
makes the already long argument list even longer.
Thought about a couple of ways to do this, unfortunately this is the only
decent way to do this that I see. There's also no way to pass operator options
currently.

Getting the handling to work correctly took some effort, and the code is not
exactly nice.

Note that even though we activate the item when calling the custom drag
operator (helps indicating the pose being blended for example), we only call
the drag operator then, not the activate one. They are never executed both.

Also note that this won't compile right now, I'll have to commit some changes
to master first.