[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.