appDisplay: Add a menu item to launch on the discrete GPU

appDisplay: Add a menu item to launch on the discrete GPU

It will only show up when a discrete GPU is available (detected through
the switcheroo-control D-Bus service), and the application hasn't
alreayd been launched.

Note that this will not currently work for D-Bus activated applications,
eg. the menu item will be available, but the environment variable will
not be passed through to D-Bus to use when launching the application.

https://bugzilla.gnome.org/show_bug.cgi?id=773117