Add global shortcuts support on MATE

src/globalshortcuts/globalshortcutsbackend-system.h

@@ -41,6 +41,8 @@ class GlobalShortcutsBackendSystem : public GlobalShortcutsBackend {
   explicit GlobalShortcutsBackendSystem(GlobalShortcutsManager *parent = nullptr);
   ~GlobalShortcutsBackendSystem() override;
 
+  bool IsAvailable() override { return true; }
+
  protected:
   bool DoRegister() override;
   void DoUnregister() override;