QDialogButtonBox does not need TabFocus

It will now share the same behavior as QGroupBox, which has NoFocus
if its not checkable.

This doesn't lure AT clients to stop and read its content (this is the
case for android accessibility) (designer seems to generate a tooltip
for each QDialogButtonBox, which will be read as the name).

Change-Id: I6cfacdd9c01299521222c773634df1e36971d982
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>