Add QObjectProperty, QPropertyAlias, QObjectPropertyAlias

Now that we have QProperty, we need some additional functionality for interoperability with old style properties, and as representation of aliases, the way QML implements them. In particular

• QObjectProperty wraps a Q_PROPERTY of a QObject, and provides the same API as QProperty.
• QPropertyAlias is an alias to a QProperty. You can get and set its value and subscribe to changes like you can do with QProperty, but you cannot attach a binding. All access is immediately redirected to the actual QProperty.
• QObjectPropertyAlias is a QPropertyAlias for a Q_PROPERTY of a QObject. It shall have the same interface as QPropertyAlias.

QPropertyAlias shall live in qproperty.h. For QObjectProperty and QObjectPropertyAlias a new header qobjectproperty.h shall be created.