Added shortcut for displaying the timetable.

* It's displayed in Firefox, so one is interested to have an instance of it
  running at the moment using the shortcut, otherwise it's going to take a *lot*
  of time.
* It's simply an image; if it doesn't exist, a 404 is displayed.
* Removed the AltGr+Return for pushing the master window on the stack.
  => too many accidental 'switchings' made me go crazy, so I'm removing that.