Merge QQmlAccessible into QAccessibleQuickItem

The idea of the separation was to keep Qt Quick 1 and 2 accessibilty in
sync. This is not going to happen because Qt Quick 1 accessibility was
never supported and will not be in the future either, so instead remove
code duplication and merge the two classes.

Change-Id: I508eb0ae20ab22ee92806a4c648c39b35259b6b9
Reviewed-by: Jan Arve Sæther <jan-arve.saether@theqtcompany.com>
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@theqtcompany.com>