When only a note is left it should be deletable and, if deleted, replaced with a new one

From last revision, the last note in a notebook can't be deleted. But it's as if the last tab of a browser can't get closed. We should let the user delete the last note and the replace it with a new one.

Maybe the user can get confused by the inability to delete the last note, so the new note could have a body like this: "You cannot have empty notebooks. If you want to delete this notebook, press the 'Remove Button' in the left bottom corner. Or you can delete this message and normally use this note", but this is just an idea.