Blender Git Loki

Gizmo: changes to internal drag logic

Minor changes to recent gizmo click/drag logic 08dff7b40bc6a

Changing the gizmos highlighted part in the invoke_prepare
callback is too error prone since it needs to run
before it's known which operator will execute.

Add back 'drag_part', since it simplifies click-drag use.
While this isn't essential with custom keymaps per gizmo
it avoids having to define a keymap in the case a drag
event needs a different action.