Refactor connection handling
- Move all signals for connection handling emitted by 'Kaidan' to itself
- Add methods for initialization after objects and threads are created
- Catch corner cases of connecting / disconnecting safely for more stability
- Load credentials from settings file by 'ClientWorker'
- Introduce functionality for running tasks upon successful login
- Change password and nickname correctly on server even if not logged in when calling to improve user experience
- Introduce 'AccountManager' for account-related settings
- Always disable automatic reconnection on connecting to server
This should be merged after !615 (merged), !616 (merged), !618 (merged), !619 (merged) and !626 (merged).