When defining custom callbacks to the undo and redo handlers, the undo
and redo callbacks were swapped. Undoing an action would call the redo
handlers and redoing an action would call the undo handlers.

The step direction was recently switched (rBc13383d) causing the
BKE_callback_exec_id call in ed_undo.c:255 and ed_undo.c:304
to fail. The ternary if operator must have slipped under the radar.

The fix checks the step direction using the new enum eUndoStepDir.