Fixes to electron desktop notifs

Merge the notification part of element-hq#2960

 * Show and focus the window when the notification is clicked,
   rather than just restoring it.
 * Implement requestNotificationPermission and return a resolved
   promise (although in practice it should never be called)

src/vector/platform/ElectronPlatform.js

@@ -97,7 +97,10 @@ export default class ElectronPlatform extends VectorBasePlatform {
                 room_id: room.roomId
             });
             global.focus();
-            electron.remote.getCurrentWindow().restore();
+            const currentWin = electron.remote.getCurrentWindow();
+            currentWin.show();
+            currentWin.restore();
+            currentWin.focus();
         };
 
         return notification;
@@ -131,4 +134,8 @@ export default class ElectronPlatform extends VectorBasePlatform {
     screenCaptureErrorString() {
         return null;
     }
+
+    requestNotificationPermission() : Promise {
+        return q('granted');
+    }
 }