Remove obsolete code from shortcut handler

These lines were introduced in 612f59aa10eef2bf6559deddfb54b84a0a30c6bc
when the only shortcuts were +/-/=/0 (to change zoom level).
Back then, when the input fields were focused, using these keys would
add these unwanted characters to the input field and change the zoom
level of the viewer. Having both operations at the same time was an
undesired event, so a check was inserted, to ignore shortcuts when
the toolbar is focused.

The #toolbar element doesn't exist any more, so these lines are obsolete.
Handle navigation keys when viewer is not focused

Restrict the scrollbar hack to the keycode for "spacebar",
since the bug only occurs in Firefox with spacebar.

Keyboard navigation is only activated for the spacebar if
the currently focused element is not a button.
Merge pull request #4224 from Rob--W/shortcut-responsiveness

Focus container when arrow/navigation keys are pressed