- reuse some code from ComboBoxField throw an abstract ListField class
 - ListField can handle more data like: icon, trToolTip
 - fix disabledIndex in ComboBoxField
 - adjust documentation

Change-Id: I00b6ab787fb2fad97dafff32786cf73c636c772d
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>