extensionDownloader: Adapt install dialog to new design

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/962

data/theme/gnome-shell-sass/widgets/_dialogs.scss

@@ -258,11 +258,6 @@
   spacing: 30px;
 }
 
-/* Extension Dialog */
-.extension-dialog {
-  .message-dialog-title { font-weight: normal; color: $fg_color; }
-}
-
 /* Inhibit-Shortcuts Dialog */
 .inhibit-shortcuts-dialog {
   spacing: 30px;

js/ui/extensionDownloader.js

@@ -197,7 +197,8 @@ class InstallExtensionDialog extends ModalDialog.ModalDialog {
         }]);
 
         let content = new Dialog.MessageDialogContent({
-            title: _("Download and install “%s” from extensions.gnome.org?").format(info.name),
+            title: _('Install Extension'),
+            description: _('Download and install “%s” from extensions.gnome.org?').format(info.name),
         });
 
         this.contentLayout.add(content);