Make it possible to add custom property editors to Designer

Designer supports custom widgets, and these widgets can also have properties declared with the Q_PROPERTY() macro. These properties can be of any type, but Designer only supports editing of types that QVariant understands, in addition to some special types like QFont and QPalette.

It would be nice if it was possible to provide custom property editors for custom types (or any type that Designer does not support by default), or even custom property editors for overriding the default editors provided by Designer.