add operator== to ARDOUR::MidiSceneChange
add color property to MIDISceneChange
serialize/deserialize MIDISceneChange color; put out of bounds color into static const
when creating a new MIDI scene change, check existing scene changes for equality and reuse their color (which may or may not be set)
refactor MIDISceneChange color property addition by moving it into SceneChange to anticipate other types of SceneChange objects (e.g. OSC)