Re-port support for syncing back labels to/from X-Keywords 

I wrote this support a long time ago and it bit rotted against current sup releases.  I've ported the patches and think this is now actually a simpler approach.

Note that I'm not sure that this will work against the currently released offlineimap version.  I know that my fork of offlineimap will work with it (https://github.com/cro/offlineimap) and I have a PR into the offlineimap project to fix an issue I found (they may very well fix it a different way).
Forgot to take that comment out.

The point was that if you are only syncing Gmail's All Mail then by default every message in your All Mail will end up with a Sup inbox tag.  So when syncing labels with X-Keywords it's better to use Gmail's \Inbox flag.