separating interface and implementation

[Roland Illig <roland illig gmx de>]

Hi all,

I would like to have some clean interfaces in the Midnight Commander. To 
realize this, I would like to split some headers (dialog.h, widget.h) 
into an interface part, which exports only the type names (WButton), but 
not the implementation (struct WButton). It would look like:

widget.h:
    typedef struct WButton WButton;

widget-impl.h:
    struct WButton { ... };

Then we can encourage ourselves not to use the *-impl.h headers where it 
is not necessary. And when only a few files do use the implementation, 
we can change it more easily, if that ever becomes necessary.

Roland