Implement station data caching and local stations

The intention of this MR is to fix issues like #572 (closed), #499 (closed) et al. by extending the database to include station metadata.

• The database uuid column becomes the primary key and is also used for identifying local stations
• A new is_local column has been added to identify local stations
• An additional data column has been added to store optional cached data (mandatory for local stations)
• The library has been adapted to handle all possible cases of database state
• The SwStation class has been extended to also provide access to uuid and is_local

This MR leaves room for implementing the user interface for local stations on top.