Annotations controller is always specified in a SessionController
Provide a mechanism for forcibly overriding a keybinding

SessionController needs to override the backspace key at all times, and
if the document is not editable, simply return true. The easiest way to
do this is to switch out the keybinding, and forcibly clobber the existing
keybinding when leaving edit mode.
Merge pull request #293 from peitschie/edit-mode-twice

Don't crash if edit-mode is entered twice
Uses the previous selection style for the newly inserted character when replacing text