PropMap callback implementation

[ghost]

Implement a flexible callback system for PropMap, supporing multiple instances with different callbacks.

Refer to:

• http://caml.inria.fr/pub/docs/manual-ocaml/intfc.html#sec424
• https://github.com/mrvn/ocaml-qt5/blob/master/src/qt5/src/Signal.h#L35
• https://github.com/mrvn/ocaml-qt5/blob/master/src/qt5/src/Connection_stubs.c

[Kakadu]

I do not really understand the description. Can you clarify this somehow @orbifx?

[ghost]

I think this issue is outdated, irrelevant now. The QML property binding is a superior approach.