more event handling fixes:

* introduce more subtle/timed logic to decide how to route wheel events to subwidgets or to the view. It works much more nicely this way as there is no more need for the user to focus an element before being able to scroll using the wheel. This is close to the Gecko behaviour, I'd describe.

* properly translate mouse wheel events (gah! that was the deeper reason for most of wheel event problems)

* remove test of the widget rectangle for mouse wheel events. As it turns out, I was just turning around the untranslated mouse wheel event bug.

* make listbox & textarea widget not propagate wheel events to the view once either end of them has been reached by scrolling.

* rewrite & factor out the code computing the widget placement on the root view; place it in KHTMLWidgetPrivate.