Updated changelog.
Bugfix: read_syx_file() didn't handle empty file.
Tiny refactoring.
Bugfix: bugfix: timing in MIDI playback was broken from 1.1.7 on.

Current time was subtracted before time stamps were converted from
ticks to seconds, leading to absurdly large delta times. (Reported by
Michelle Thompson.)
Rewrote delta time computation to make the code a bit clearer.
Renamed method for clarity.
Some cleanup.
Bumped version to 1.1.8.