Handle client state a bit more sanely

Handle client state a bit more sanely

Client can only be disconnected or connected. In the latter state, we guarantee
that the typical connection-specific (syncable) objects at least exist (but may not
be fully synced yet).

It doesn't make much sense API-wise to distinguish between "connected" and "synced" state
of the Client object, as vital objects are created after being connected. In any case,
more detailed information is available via Client::coreConnection().