Prevent the backspace key from causing the browser to go back in history

The recent extraction of text manipulation functions slightly changed
the backspace key behaviour. The new interface was only returning true
if an operation was generated as a result of the call (aligning with
all other public functions on the class). This means however that back-
spacing when there is no available positions (e.g., at the beginning of
a document) was causing the browser to navigate back a page.
Allow public access to the SessionController's keyboard handler

This is the first step towards allowing external users to modify (or
disable) the built-in webodf keyboard shortcuts.