[Feat] Store list of databases in settings

[wenzeslaus]

Is your feature request related to a problem? Please describe.

The database directories can be added to the Data tab, but they are not remembered next time I run GRASS GIS. When I'm using several databases, I need to add the other ones again.

On the other hand, once I add a database, it cannot be removed. This issue will be more important once the databases are remembered.

Describe the solution you'd like

The GUI should remember the databases added. Every time a new database is added, a list of databases in the user settings should be updated. This way, a new GUI instance should read the user settings and put the databases to the list (if the directories still exist).

The GUI should allow to remove the database from the tree. This should again be reflected in the user settings.

Describe alternatives you've considered

Alternatives to storing the list of databases from the Data tab in the user settings is 1) storing them in a workspace file (that can be added), 2) not storing them at all (current state, but not good for users with multiple databases, e.g. one for each project), 3) tracking recently used ones (less flexible and impossible for the user to customize/directly influence), and 4) track all ever used (like the previous, but with no limit).

Additional context

This was discussed on Next steps in GSoC and beyond video call, July 15, 2020 (notes).

The partially related need for a remove button brings the question of an actual delete button. That makes sense too, but it is outside of scope of this issue.