unlockDialog, loginDialog: Add a 'wake-up-screen' signal

The signal is currently present in the notifications box, but next
commits will move the notifications box to the unlock dialog.

Add a 'wake-up-screen' signal to the dialogs.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/872

js/ui/screenShield.js

@@ -819,6 +819,8 @@ var ScreenShield = class {
             }
 
             this._dialog.connect('failed', this._onUnlockFailed.bind(this));
+            this._wakeUpScreenId = this._dialog.connect(
+                'wake-up-screen', this._wakeUpScreen.bind(this));
         }
 
         this._dialog.allowCancel = allowCancel;