shell/global: Add 'backend' property

Replace all Meta.get_backend() with global.backend. Maybe sooner or
later we can further decrease the amount of singletons handled by
libmutter.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1210

js/ui/keyboard.js

@@ -1119,7 +1119,7 @@ var KeyboardManager = class KeyBoardManager {
         this._seat.connect('notify::touch-mode', this._syncEnabled.bind(this));
 
         this._lastDevice = null;
-        Meta.get_backend().connect('last-device-changed', (backend, device) => {
+        global.backend.connect('last-device-changed', (backend, device) => {
             if (device.device_type === Clutter.InputDeviceType.KEYBOARD_DEVICE)
                 return;