unsynced: add unsynced flag to keyrings

Closes #533

The design is rather... uh... basic at this point. I pushed the message around a bit, but it never really fit all too well.

Unlike #570, this is a very simple database update which we could just as well add at a later point if we decide the feature doesn't fit in the ui right now (or ever), and the code is of the type to age fairly well so this is not as important for the OK 2.6 milestone as originally thought.

Comments please.