Add signal to Account for logged in

[TheOneRing]

AccountState has the logged in state but this is only available in the ui code. The spaces manager misuse the credentialsFetched signal but thats a bit hacky.

[erikjv]

I think it would be better to either move the SpacesManager to the AccountState (as it's only used when there is a state), or connect it (directly, or indirectly through a lambda) to the AccountState::stateChanged signal.

[erikjv]

The SpacesManager is used in the cmdline client, so moving its ownership is not an option. Going for #2: connect the slot from the outside when creating an AccountState