Code review comment for lp:~mterry/ubiquity/plugins

This is phase 1 of my plugins work. I'm 3/4 of the way there, I think. The basic infrastructure is there, I've fully converted the first four pages of ubiquity to plugins (intro, language, location, keyboard) and half-converted the mythbuntu frontend to a series of plugins. The other ubiquity pages I've modified just enough to still work in the new scheme.

Slightly outdated spec about this work here: https://wiki.ubuntu.com/Ubiquity/Plugins . I intend to update that to match what this tree has shortly (tomorrow), but I wanted to get this merge request in, and maybe some eyeballs, before I finished that.

All the frontends should work with the new plugins. A plugin has to explicitly support a given frontend, so this let us much more elegantly handle the case of some frontends (notably debconf) supporting a different set of components than the others. All in all, hopefully 'elegant' is the watchword.

There should be no user-visible changes from this branch.