Avoid masking addChild, removeChild with treeMixin.
When deleting a book, avoid re-editing the same book.

This happens because the removeMe call becomes async on account of an
on-the-fly dependency to epub-container causing the delete to happen in
a callback, which means that when we select the next book to open, the old
one hasn't been deleted yet.
Hide the module title when editor is closed.

This includes a small improvement where the editor element is find through
traversal from @$el rather than a global jquery search. It also solves the
problem where deleting a module/book leaves the title hanging.
Merge remote-tracking branch 'origin/master' into delete-first-book-P62447308

Conflicts:
	scripts/views/workspace/content/layouts/editor.coffee
Merge pull request #129 from oerpub/delete-first-book-P62447308

Update Editor title when deleting a book, handle deletion of first book.