getModifierSide method
- ModifierKey key
override
    Returns a KeyboardSide enum value that describes which side or sides of the given keyboard modifier key were pressed at the time of this event.
Implementation
@override
KeyboardSide getModifierSide(ModifierKey key) {
  // Neither GTK nor X11 provide a distinction between left and right
  // modifiers, so defaults to KeyboardSide.all.
  // https://code.woboq.org/qt5/include/X11/X.h.html#_M/ShiftMask
  return KeyboardSide.all;
}