Fix icon on empty notification panel (#10817)

* Use custom properties for the empty panel icons on FilePanel and NotificationPanel

* Create E2E test for the notification panel

res/css/structures/_FilePanel.pcss

@@ -113,5 +113,5 @@ limitations under the License.
 }
 
 .mx_FilePanel_empty::before {
-    mask-image: url("$(res)/img/element-icons/room/files.svg");
+    --maskImage: url("$(res)/img/element-icons/room/files.svg"); /* See: _RightPanel.pcss */
 }

res/css/structures/_NotificationPanel.pcss

@@ -0,0 +1,19 @@
+/*
+Copyright 2015, 2016 OpenMarket Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+.mx_NotificationPanel_empty::before {
+    --maskImage: url("$(res)/img/element-icons/notifications.svg"); /* See: _RightPanel.pcss */
+}

res/css/structures/_RightPanel.pcss

@@ -179,6 +179,7 @@ $pulse-color: $alert;
         height: 42px;
         width: 42px;
         background-color: $header-panel-text-primary-color;
+        mask-image: var(--maskImage);
         mask-repeat: no-repeat;
         mask-size: contain;
         mask-position: center;