Fix for ticket #2942: character counter now updates on cut and paste operations made...

Fix for ticket #2942: character counter now updates on cut and paste operations made with mouse or menu

This uses the 'copy' and 'paste' DOM events to trigger a counter update. I haven't had a chance to 100% confirm that middle-button click on X11 triggers the event, but it ought to.
Cut and paste events from context menu and main edit menu known good in:
* Firefox 4.08b-pre
* IE 9 preview 7
* IE 8 current
* Chrome 8 beta current
* Safari 5.0.3

Opera is listed as not supporting these events, oh well.

Note that using a *delete* command from a menu doesn't trigger an event. Sigh, you can't win everything.