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/modal.html

@@ -13,7 +13,7 @@
           <div class="panel-icon">
             <span ng-if="icon" class="fa fa-{{icon}}"></span>
           </div>
-          {{title}}
+          {{heading}}
         </h4>
       </div>
       <div class="modal-body" ng-transclude>