Add proper key navigation to layout based window switchers

The grabbed key events inside TabBox are forwarded to the declarative
view which passes it to the normal keyPressedEvent() method. This
allows the QML files to handle keyboard navigation themselves.

The views support key navigation natively, though this cannot be
used as we need to update the ModelIndex when a new item is selected.
Also there seems to be a problem if the tabbox is shown again, in
that case the focus seems to be somehow lost. Because of that the
navigation is handled in the Item embedding the list.

REVIEW: 104357

FIXED-IN: 4.9.0