Handle digital multiplexes in radio tables

[TomaszGasior]

This is technical description of solution for problem described in this discussion: #211

Introduce ability to add multiplexes and digital radio stations (inside that multiplexes) into radio table, as completely separate entities from analog radio station. Currently radio table contains just radio stations. Here I want to change this so radio table can contain analog radio stations and digital multiplexes (with digital radio stations inside).