Next release

[Adam Fedor]

It's been a long time now (LTN) since our last release, perhaps we  
should do a new one soon?   Also, in an effort to please everyone  
(ETPE), I was thinking we should separate the release numbering from  
the SO version numbering, at least on the unstable branch as long as  
there are no ABI changes.  This should be really easy as we just need  
to define

INTERFACE_VERSION

in the top-level makefile.  So

Base/Gui unstable release:
  Subminor release (1.19.X) - bug fixes or API changes
        increase subminor version
        interface version does not change
 Minor release (1.X.0) - API changes and bug fixes
        increase minor version
        interface version does not change
  Minor or Major release (1.X.0 or X.0.0) - ABI and API changes
        increase version
        increase interface version to match.

Base/Gui stable release:
  Subminor release (1.18.X) - bug fixes only
        increase subminor version
        interface version does not change
 Minor release (1.X.0) - ABI, API changes and bug fixes
        increase minor version
        interface version changes to match
  Minor or Major release (1.X.0 or X.0.0) - ABI and API changes
        increase version
        increase interface version to match.