Prevent "calling draggable before initailization" errors

Elastic skin disables dragging on folders list, so we have to check
if it's still draggable before we call draggable() again. Otherwise
it will throw an error.

program/js/app.js

@@ -7121,7 +7121,9 @@ function rcube_webmail()
     }
 
     // disable drag-n-drop temporarily
-    this.subscription_list.draggable('destroy').droppable('destroy');
+    // some skins disable dragging in mobile mode, so we have to check if it is still draggable
+    if (this.subscription_list.is_draggable())
+      this.subscription_list.draggable('destroy').droppable('destroy');
 
     var row, n, tmp, tmp_name, rowid, collator, pos, p, parent = '',
       folders = [], list = [], slist = [],

program/js/treelist.js

@@ -89,6 +89,7 @@ function rcube_treelist_widget(node, p)
   this.intersects = intersects;
   this.droppable = droppable;
   this.draggable = draggable;
+  this.is_draggable = is_draggable;
   this.update = update_node;
   this.insert = insert;
   this.remove = remove;
@@ -1245,6 +1246,11 @@ function rcube_treelist_widget(node, p)
 
     return this;
   }
+
+  function is_draggable()
+  {
+    return !!ui_draggable;
+  }
 }
 
 // use event processing functions from Roundcube's rcube_event_engine