pyface.api should expose interface classes

[rahulporuri]

For example, pyface.api does not re-export the IGUI interface class but it exports the GUI implementation/provider.

[corranwebster]

It should; in fact it possibly should expose them separately to the main api.py as well so people can access the interfaces without needing to also import the implementations (and do toolkit selection, etc.). Right now if they want that they have to import from the modules where the interfaces are defined.

[rahulporuri]

another example that was observed recently was IDropHandler.