Improve DropDown keyboard behavior

[grokys]

DropDown needs a bit of work to match the standard ComboBox keyboard behavior on Windows:

• Arrow keys should change selection when popup not open
• Changing selection in popup with keyboard shouldn't update selection until Enter pressed
• Pressing Down when no selection and popup open should select first item

[jkoritzinsky]

How much of this was fixed by #1166?

[grokys]

@jkoritzinsky i think the current status of this issue is correct - just the second item needs addressing.

[jkoritzinsky]

Ok. I'm going to push the last part to Beta2 since it's not a bug or a perf-related feature.