The `CategorySelector` was possibly added to be incremented with other
attributes. However, since none other that the selector enum is used at
the moment, it is only adding unnecessary cluter.
The category selector value is moved to the parent
`TransactionCategory`.
`add` method replaced with `insert`.
`insert` and `select` implemented for new database base class.
Database unit test added.
Due to SQLite implementation of the primary key autoinc, the type of the
IDs on the database for SQLite changed to Integer.
https://www.sqlite.org/autoinc.html
