Blender Git Loki

Outliner: Keep header operator-free and edit menu cleanup

Headers should not have operators as much as possible. The exception here is
for datablocks mode when you want to see the active keyset.

Edit menus on the other hand should be clearly distinct from the RMB context
menus. Edit menu options should be only the ones that apply to the entire
outliner, regardless of the selected element.

Context (rmb) menus should be related to the element you RMB on to invoke the
menu. I'm also taking this opportunity to start bringing the context menus
to Python. There is little reason not to, and it helps editing them (In this
case I'm doing it only for the Scene Collection one).