Make undo work for changes to MWExtensionNodes

If you changed the contents of an alien tag extension, then tried to
use undo, the change wouldn't be undone, but the undo state would
toggle.

This was because we only kept one copy of the data-mw object for these
nodes and modified that one copy. Instead, create a new copy every time
we modify it, so we can actually go back to previous states.

Change-Id: Ib4d5d460a6fdda9c3b0968f269585d620b47fdd8
Merge "Make undo work for changes to MWExtensionNodes"