Callbacks gtk-Java and gtk-D
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SWT uses a Mechanism to make callback possible from C -> Java. This mechanism is
implemented in swt/internal/Callback.java and the C libs. It can convert a delegate call
into a simple function pointer.

This works by generating a whole lot of function wrappers. All with int argument types,
varying in argument count. The Callback class implements a registry for the available
functions and redirect the C call to Java using reflection.

In the D port, dwt.widgets.Display has the struct CallbackData. It wraps the instance pointer
to the display instance. If a callback uses other userdata, it is wrapped also into the
struct.

Caution by g_signal_handlers_[un]block_matches: In Java sometime integer constants are passed
to select a callback for enabling/disabling. In D, the Widget class has getter function to get
a pointer to the CallbackData that was used instead of the integer when registering the
callback.