Minor changes to recent gizmo click/drag logic 08dff7b40b
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.