Fixed a bug on undoing removal of last assistant
Undoing the removal of the last assistant will not make the removed assistant show up again. This is due to the removal of the last assistant in `KisPaintingAssistantsDecoration` will `setVisible(false)`, but the undo command has yet to revert this. This commit involves API changes in `KisPaintingAssistantsDecoration`.
Showing with 13 additions and 1 deletion