Support reading and writing of CATEGORIES.
Reading can be done with folks, writing cannot because there is no detail defined for it:
No upstream bug for it at the moment. Need to create one once we have a patch.
I've got patches for SyncEvolution and folks. Will test and commit later.
Fixed in folks (https://bugzilla.gnome.org/show_bug.cgi?id=693333) and SyncEvolution.