Settings refactoring

[co-stig]

1. Replace Qt Settings with something neutral, ideally OOTB,
2. Split settings into groups (corresponding to tabs?)
3. Core groups will live in the Core module, UI stuff will be part of "qt" module
4. Provisioning for connection- and plugin-specific settings, e.g. websocket settings
5. Make JSON settings like last used Work Summary configuration, or tutorial settings a proper groups instead

Related to #104.