gui: Remove tooltip shown over whole modal

Changed the attribute 'title' (reserved) to 'heading' for the modal
template

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3281

gui/default/syncthing/core/shutdownDialogView.html

@@ -1,3 +1,3 @@
-<modal id="shutdown" status="success" icon="power-off" title="{{'Shutdown Complete' | translate}}">
+<modal id="shutdown" status="success" icon="power-off" heading="{{'Shutdown Complete' | translate}}">
 	<p translate>Syncthing has been shut down.</p>
 </modal>