Store events and accounts locally and adapt to changes

This makes the library stateful, but users of the library shall have lesser hassles on how encryption/decryption and any other internal stuff will be handled, plus it allows as to expose even less code.

The events are now stored in the sqlite database. Currently all events are stored, but only message events are retrieved when requested (as that's all is what chatty know)

The account secrets are now stored using libsecret (we use app-id.CMatrix domain)

This is a massive change and thus will likely break stuff (again, sorry. But that's the reason I haven't yet made a chatty release). All except the major issues shall be fixed in subsequent merge requests.

This shall be the last planned breaking change to happen in libcmatrix

/cc @guido.gunther