Music 5000 with Filtering

This is a version of B-Em in which the Music 5000 emulator has an output filter. The default is a filter which approximately matches the one in the original hardware. Another option is a 16kHz filter as used by the jasoni remake (see https://stardot.org.uk/forums/viewtopic.php?p=235383#p235383). If neither of these options is selected, no filtering is done.