GUI interfaces for gt-api

Creating this issue as a note to myself, Jody and Moovida...

Part of the recent work on the swing module has been to extract interfaces, primarily for unit testing purposes but also with an eye to having a common base for the gt-swing and gt-swt modules (and perhaps other GUI efforts in the future).

The attached test file (formatted as rst in the hope that we might pillage it for docs later) lists the existing interfaces and the classes that implement them in gt-swing. I'll update this as work progresses. Please feel free to edit this as you see fit or add other docs, wish-list items, comments etc.