gir: Make use of typedefs for GimpProgressVtable vfuncs
GObject introspection has issues handling random vfuncs (i.e. not part
of a GObject). Most of all, it needs to be able to give these a name
(so they can be used to cast in some higher level languages for
example). As such, give each vfunc in GimpProgressVtable
a separate
typedef.