Blender Git Loki

Fix T81589: Correct drag type handling in outliner

Blender crashed when dragging and dropping color into the outliner.
This issue was cause by a missing check for the correct drag type
in `datastack_drop_poll`. The check is added in this commit.
Additionally, a new drag type is introduced for the "data stack"
drag option, that was introduced in commit 1572da858df4, to
differentiate it from the existing WM_DRAG_ID type.

Reviewed By: Severin

Differential Revision: https://developer.blender.org/D9169