Make it possible to handle pointers to QObject derived in QML.

Make it possible to handle pointers to QObject derived in QML.

This way, properties of QObject derived types can be read in QML
code for example:

 Q_PROPERTY(MyObject* obj READ obj CONSTANT)

Previously, only QObject* types could be read by QML:

 Q_PROPERTY(QObject* obj READ obj CONSTANT)

This meant that multiple properties and methods had to be created
for classes which were relevant to both QML and non-QML code.

This patch lifts that restriction.

As a consequence, we can also remove a Q_EXPECT_FAIL from
the qqmllanguage unit test. That test was introduced in
commit 92562eacbc3c (Allow signal parameters which are custom QML
object-types, 2012-07-13) to document knowledge of the limitation
while fixing it as much as possible.

Task-number: QTBUG-26662

Change-Id: Ic85fa73c6f3655189438ec509765bae2eab9993a
Reviewed-by: Alan Alpert (Personal) <416365416c@gmail.com>
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>