While Roundup can be considered to be a framework to build issue
trackers, its APIs aren't really componentized in a way that makes
defining plugins easy. Thus it becomes unwieldy to add reusable extensions.

Try to adjust the existing APIs such that it becomes possible to define
plugins that encapsulate self-contained sets of functionality, which are
then portable from one tracker instance to another, allowing users to
build extensions and then package and share them.

(A good use-case for this may be support for versioning control. We
tried to get this done in a previous GSoC project, but unfortunately the
project failed.)

I tentatively mark this issue 'GSOC', even though it may actually be too
big for one. This needs further discussion and refinement...