Tighten up conceptual model for (our) 2.0

[bitprophet]

After #45 it's become more apparent some things are a bit wonky.

The first one that comes to mind is that the major/backported keywords aren't consistent - a 'backported' feature or support item gets put in both bugfix releases and feature releases, but 'major' bugs are only considered "feature-like" and live in feature releases only.

Given the way we treat bugfix/tertiary releases - "feature releases are assumed to include prior bugfixes implicitly" - this seems incorrect. Either that assumption holds or it doesn't. My inclination is that my earlier inclination about 'backported' feature-like issues appearing in both types, is wrong, and all issues should either get displayed "bug-like" or "feature-like" and never both. As-is it makes the changelogs messier and, as noted, is mentally inconsistent.

However changing this would be backwards incompatible (oh the irony) so we'd want there to be a 2.0 release with this sort of change.

Also a good time to more strongly do away with backported/major or e.g. make them distinct issue types of their own like :major-bug: or :minor-feature (though maybe better terms would be nice since that's confusing wrt major/minor release numbering...heh).

And of course, a good excuse to continue the internals cleanup started with #45.

[MinchinWeb]

Rather than hard defining :major-bug: and :minor-feature:, would it make sense (or is it even possible) to arbitrary labels? So at the top of your changelog, have an RST directive like:

.. changelog-role: major-bug
    :display-string: Bug
    :lines: minor

.. changelog-role: breaking-feature
    :display-string: Bug
    :lines: major

.. changelog-role: gui
    :display-string: GUI
    :lines: patch, minor, major

[MinchinWeb]

If we're talking breaking changes, would it make sense to be consistent with SemVer in the use of "major", "minor", and "patch"?

So if the project is currently at 1.2.0, :bug:major would be added to the 2.0.0 release, but not 1.3.0 or 1.2.1 (because 2.0.0 is the next "major" release).