ConfigData: update API documentation
decoder/sidplay: make "songlength_file" local
decoder/sidplay: use config_param::GetBlockPath()
decoder/sidplay: use class SidDatabase

Remove our own songlength database parser.
decoder/sidplay: include cleanup
decoder/sidplay: simplify the SidDatabase::length() call
decoder/sidplay: pass SidTuneMod to get_song_length()

Eliminate duplicate SidTune construction.