[FIX] Different exception is script could not be executed because the browser was disposed
[FEATURE,UI] If elements are selected while a tree is filtered, the previous expansion state restored. New: Also the elements that were selected elements in the filter state are restored.

Example: Elements A and B are expanded. The user filters for C and selects it. Then he resets the filter. Now the elements A, B and C are expanded.
[FIX] Made composite field of AbstractDemo private so Demos can no longer accidently add children to it
[FIX] [droppable].over CSS class is now correctly set
[FEATURE] The DNDListener now also gets the dragged/dropped element