Allow AppKeys key bindings in Overview mode.

The proposed modification allows AppKeys to work in Overview mode. This capability was also mentioned in Pull request [#11](https://github.com/franziskuskiefer/app-keys-gnome-shell-extension/pull/11#issuecomment-59417085).

The other option would be to add that 'capability' in Preferences -- I'm not actually sure if anyone would want the functionality disabled in Overview?

It works with Gnome 3.10, 3.12, 3.14.