shell-app: Add "discrete_gpu" option when launching apps

And adapt existing callers to the new API. This will allow us to
implement a way to launch applications on the discrete GPU for systems
where an "Optimus" system exists.

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

js/misc/util.js

@@ -94,7 +94,7 @@ function spawnApp(argv) {
                                                       Gio.AppInfoCreateFlags.SUPPORTS_STARTUP_NOTIFICATION);
 
         let context = global.create_app_launch_context(0, -1);
-        app.launch([], context);
+        app.launch([], context, false);
     } catch(err) {
         _handleSpawnError(argv[0], err);
     }