Added a new keyboard shortcut named unmaximize_or_minimize_window_key.

This key will restore a maximized or semi-maximized window and minimize a restored window.

Per default this shortcut is disabled and has yet to be enabled via ubuntu-config.patch
for the Ubuntu distribution and Unity.

(LP: #966099). Fixes: https://bugs.launchpad.net/bugs/966099.

Approved by Brandon Schaefer.