Selectmenu ARIA tweaks

For code review, to eventually merge into the selectmenu branch. I've pushed the individual commits for now, we can squash that into one later.

There are currently a few tests failing - I haven't updated those yet.

I've tested with JAWS and NVDA under Windows and VoiceOver under OSX.

* NVDA/Firefox: Good
* NVDA/IE9: Good
* JAWS/Firefox: Good
* JAWS/IE9: Works, but value is announced two times when changing it
* VoiceOver/Safari: Unusable. Gets announced as combobox, but nothing else happens, at all. Native select is announced as "popup button" instead. Sucks, but then I also don't understand how anyone would use VO with Safari... I guess its good for native apps?

Feature wise this is probably as good as it gets, for now anyway. We can still improve later, but this is already much better then what we had.

Once the code is reviewed I'll sqaush and fix the unit tests.