Publically exposing GtkFileChooserIface

[Kristian Rietveld <kris imendio com>]

Hi Federico,

It has come to our attention that important pieces needed to implement a
third-party file chooser dialog outside of GTK+ are not installed.  As far as
I can see implementing your own file chooser dialog involves implementing the
GtkFileChooserIface and then plugging it into GTK+.  The main problem is that
the definition of this interface is in gtkfilechooserprivate.h, which is
obviously not installed.  A second problem is that the definition uses types
defined in the semi-public gtkfilesystem.h (especially GtkFilePath).

The easiest way to solve this is probably to:
 * Move GtkFileChooserIface into it's own, new, header file.
   gtkfilechooserprivate.h also contains some data structures internal to
   the implementation of the default file chooser which don't make sense
   to install.
 * Add GTK_FILE_SYSTEM_ENABLE_UNSUPPORTED defines like in gtkfilesystem.h.
 * Get GTK+ to install it.


I would really like to get this into 2.12.  Objections?  Suggestions?


thanks,

-kris.