Update listening to token changes in saver

We don't need to define a watch behind - getting notified
on a `tokenChange` event is all it takes.
Broadcast a tokenChange event when gloss changes

This is the right API to use for this, not the saver directly.
This way also the navigator can mark the current chunk as changed -
only this way a change to the gloss (when nothing else is done) is
properly recorded.
Allow to delete a gloss
Merge pull request #546 from latin-language-toolkit/more_lemma_trans_fix

Another lemma translation fix
Notify user when undo/redo is triggered
Add translation for undo/redo events