ComboBox: Add MaxLength
ComboBox: Only pre-select single list item if not editable
ComboBox: Slightly increase MinSizeHint width
ComboBox: Make validators work when editable
ComboBox: Fix index out of bounds bug introduced in previous commit
ComboBox: Fix another bug

This one would happen if an editable ComboBox had a current index > -1, then the user would edit the text, then change focus to another widget. At that point the ComboBox would reset to the previous current index.
ListBox: Fix handling of index -1 in SetCurrentIndex
ListBox: Add HasCurrentItem property
ComboBox: Don't leak window handle if widget creation fails