DClosure implementation

[dhasenan]

There are a number of API functions that accept a gobject.Closure and don't accept a function/callback pair. For example, gtk.AccelGroup.AccelGroup.connect. A few problems with this:

• D's type system doesn't let me use gobject.CClosure in these locations.
• The natural way to write these in D is with a delegate. This isn't an option here.
• I can't access parameters with a gobject.CClosure in any obvious way.
• I have to cast from eg void function(void* context) to void function(), which is not type-safe.

It would be great to add a way to create an appropriate Closure object.

[MikeWey]

It look like we need to implement a marshal function to translate an array of GValues into a D function/delegate call. And add a DClosure to hold the function pointer/delegate.