interface + mixin combination

[Bill Baxter]

Looking at Lutger's new sigslot code, it occurred to me that this is 
probably a pretty common (and useful) combination:

> class Receiver : ISlotTracking
> {
>     void handleClick() { writefln("button clicked"); }
>     mixin SlotTracking;
> }

I.e. using both a) derivation from an interface and b) a mixin.

I've seen similar C++ GUI libraries, where a macro takes the place of 
the mixin.   Examples I know of include

Qt - derive from QObject and use the QOBJECT macro
wx - derive from wxObject and use the DECLARE_CLASS macro
FOX - derive from FXObject and use FXDECLARE macro

It's a useful combo which is similar to regular class inheritance, but 
different in that the implementation/behavior is defined locally and 
non-virtually rather than being inherited from the base class.

The obvious syntax for it would be to just allow 'mixin' to appear in an 
interface.

I.e. for Lutger's code:
interface ISlotTracking
{
	void _registerCallback(void delegate(Object));
	void _removeCallbacksFrom(Object object);
	mixin SlotTracking;
}

It's basically a replacement for multiple inheritance.

Would it be useful?  Any issues I've overlooked?

--bb