Blender Git Loki

FModifiers: "Add Modifiers" in Graph/NLA Editor buttons now uses a dropdown instead of a button

This brings the UI here more in line with the Constraints and Modifiers UI's.

TODO:
* The tooltips/descriptions on the operators now need fixing, as we can no longer
use custom tooltips when defining these buttons.
* The operators need to be fixed to only operate on the active data