ProTracker 2 clone v1.15

• Bugfix: The EFx (Invert Loop) effect didn't work on the whole sample loop, but
  only on the half of it.
• Bugfix: The quadrascope didn't show the volume changes from the 7xy (Tremolo)
  effect.
• Windows bugfix: Certain key modifiers didn't work (bug appeared in v1.13)
• The "real VU-meters" mode now acts more like real VU-meters by showing average
  amplitudes instead of peak amplitudes. This mode can be toggled on/off by
  pressing ALT+F11 or changing a setting in protracker.ini.
• The quadrascope code has been refactored a little bit to be slightly simpler
• Some other cosmetic changes to the code (to better match the PT source code,
  which makes it easier to verify that it's correct).