Introduce explicit id field to Settings

... and use it to set the application id of the window on Unix systems, instead of relying on the title of the application.

glutin/src/application.rs

@@ -58,6 +58,7 @@ where
                 &application.title(),
                 application.mode(),
                 event_loop.primary_monitor(),
+                settings.id,
             )
             .with_menu(Some(conversion::menu(&application.menu())));