DWT event handling

[Frank Benoit]

I added the template function Bill suggested once to the 
dwt.widgets.Listener module.

http://www.dsource.org/projects/dwt-linux/browser/dwt/widgets/Listener.d?rev=243%3A84629474b5ec

You can see that in action in this snippet
http://www.dsource.org/projects/dwt-samples/browser/snippets/treeeditor/Snippet111.d?rev=85%3Afa286c85e7b8

See lines: 102..106

Thanks Bill for the snippets and for this cool template function suggestion.

Frank