The copy constructor of the Layer class didn't do a copy
of the frames storage (DNA) and only a copy of the frames map (runtime).
This is fine, but we need to make sure to tag the frames storage,
because it is out of sync otherwise.
During edit mode undo, the layers were copied (but the frames storage not tagged) which meant that after undoing and then saving the file,
the frames would be gone after reloading.
The fix makes sure we tag the frames storage, so that it is properly synced.
bab3e5c442