[ComboBox] Major overhaul

Padding is handled more like Qt QQC2 styles. Doing it in other ways is
more trouble than it's worth for compatibility and getting correct
behavior.

delete commented out mouse area. not needed.

fix indicator size

add special pressed background and separator for indicator when editable

Use Menu for popup and use animated highlight. Also use large menu size
if combobox width isn't wide enough.

remove font style changes for delegate and change implicit width
calculation.

Add WheelHandler to make scrolling the mouse wheel change the index.

add code for getting the width of the widest element in the model, but
don't use right now.