Emit the highlighted signal if the model changes.

Emit the highlighted signal if the model changes.

The bug is that the connection to emit that signal can be made
obsolete if the connection is made too early and the model is replaced.
In the bug report, the connection is made by calling view() early (thereby
causing the creation of a view and a QItemSelectionModel which operates on
the built-in QItemSelectionModel, and then connecting to that
QItemSelectionModel), and then when QComboBox::setModel() is called later
the built-in view creates a new QItemSelectionModel for it. The bug was
that that new QItemSelectionModel is not connected to. This patch fixes that
bug.

Task-number: QTBUG-4454
Change-Id: Ibbdb8731f16ab071008b4a19dc2cc7ae03cebc84
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>