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).