Reminder: Implement the support for XMF files at the built-in sequencer

[Wohlstand]

As was talked here https://github.com/Wohlstand/libADLMIDI/issues/294#event-19529902956, I received an XMF file as a sample. Actaully, every XMF file is basically a container that contains normal MIDI files inside: it's possible to store multiple different MIDI files inside one XMF file (a kind of different variants of the same melody) and other content like DLS banks, etc. Primarily, it's need to parse the XMF file and extract MIDI data, and if many MIDI entries stored, treat them as separated songs in a manner of XMI.