Report value changes for focused IE6 MSHTML combo boxes

[jteh]

When an MSHTML combo box is focused and the selected item is changed, two events are fired:

• valueChange is fired on a combo box. However, the combo box seems to be some sort of proxy and does not have the same event params as the focused combo box. Therefore, we currently ignore it.
• selection is fired on a list item representing the selected item. If the combo box is closed, this list item is invisible. If open, it is visible. We currently redirect IAccessible selection events to stateChange.

We should special case one of these so that combo box value changes will be reported.

[jteh]

Forgot to note that this only applies to IE6. IE7 and later fire proper focus/valueChange events for combo boxes.

[jteh]

Implemented in r3372.