Remove QgsAbstractProperty and subclasses, and instead use a single
QgsProperty class which covers the use of QgsAbstractProperty
and its subclasses. This simplifies the API and avoids pointer
handling. QgsProperty is implicitly shared for memory efficiency
and inexpensive copies.
Instead of defining the suitable field types and values for
properties when registering them to a data defined button,
now properties are fully defined when the valid
property keys are defined.
