Refactor MED event reader

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Add Dennis Lindroos Soundtracker notes

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Hack: Adjust period for IFFOCT samples

IFFOCT samples have, apparently, periods twice as large as their regular
sample counterparts. We don't have a period adjustment infrastructure in
libxmp 4, so we'll add this hack to play certain OctaMED modules correctly
(see iainonics.mmd1).

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Fix OctaMED vibrato effect depth

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>