Class to implement icon overlays for source controlled files.
+ Specialized classes are created for each overlay icon.
Displays a different icon based on version control status.
NOTE: The system allocates only 15 slots in _total_ for all
icon overlays; we (will) use 6, tortoisecvs uses 7... not a good
- recipe for a happy system.
+ recipe for a happy system. By utilizing the TortoiseOverlay.dll
+ we can share overlay slots with the other tortoises.