Symbolic names for MIDI ports

MIDI ports shall have a user-defined symbolic name (like "Korg" or "Yamaha DX 7"). The mapping between these symbolic names and the hardware port (like "ALSA midi out port") is stored in the global configuration.

Song files only specify the symbolic names as the ports associated with their tracks. No information about physical devices/port names, but only symbolic names are stored in the song file.

This resolves the issues mentioned in 1.2, and also allows the user to share his pieces with other people: They would only have to set up that symbolic-to-hardware mapping once (collisions are unlikely, because an equal symbolic name should usually mean the same device) and are happy, instead of having to re-map every port for every song.