Property binding API

[jwharm]

Add an api to create property bindings with transformations. As an example, it seems Gtk-rs implemented some kind of Builder class for property bindings.

Currently java-gi users who want to bind properties with transform functions, must create callback methods with GValue parameters, and read/write from and to those GValues.

This probably can work with a generic builder class with <L, R> and <R, L> transformation functions that use the java-gi ValueUtil class to read from/to a GValue and cast to R or L.