Prevent folder selection changes if app is busy (#1490158)

program/js/app.js

@@ -574,6 +574,7 @@ function rcube_webmail()
       this.treelist
         .addEventListener('collapse', function(node) { ref.folder_collapsed(node) })
         .addEventListener('expand', function(node) { ref.folder_collapsed(node) })
+        .addEventListener('beforeselect', function(node) { return !ref.busy; })
         .addEventListener('select', function(node) { ref.triggerEvent('selectfolder', { folder:node.id, prefix:'rcmli' }) });
     }
 

program/js/treelist.js

@@ -248,6 +248,11 @@ function rcube_treelist_widget(node, p)
    */
   function select(id)
   {
+    // allow subscribes to prevent selection change
+    if (me.triggerEvent('beforeselect', indexbyid[id]) === false) {
+      return;
+    }
+
     if (selection) {
       id2dom(selection, true).removeClass('selected').removeAttr('aria-selected');
       if (search_active)