Add multiple favorite lists / categories

[johan12345]

e.g to group stations for a specific trip

Would need some restructuring of the database schema (new table favorites which includes just a reference to the charger ID in the chargelocations table and a reference to the category).

[johan12345]

The groundwork for this has been laid with #157, which added a separate favorites table to the database independent of the ChargeLocation table. Now the remaining task would be to add categories/tags to these favorite entries and implement a UI.

Idea for the UI: add sub-items below the "Favorites" item in navigation drawer for each category. Similar to Google Keep app.