Readonly properties support

[ChALkeR]

See http://doc.qt.io/qt-5/qtqml-syntax-objectattributes.html#read-only-properties

The testcase is failing atm.

[ChALkeR]

Not sure if that should be a separate element or a modifier of qmlpropdef.

A separate element will require another patch in QmlWeb to be supported.
A modifier perhaps could work without patches on QmlWeb side (without actually enforcing the «readonly» rule), but I am not sure how a modifier should look like here.

[machinekoder]

The readonly property is important to stay compatible with Qt QML. I would implement it using a modifier for qmlpropdef. I don't think a separate element is necessary.