Pre-release 1.6.0-alpha1

Changes relative to v.1.5.0:

• Add prototype of bird tweet instrument (#75)
• Add simple HUD with progress fillbar and song title (#82)
• Add total song time in debug display
• Make space laser not intersect through stage
• Fix a bug where music box's notes would be out of sync with actual time (#84)
• Refactor XML data to JSON for easier parsing (with kotlinx.serialization)
• Refactor MidiFile.java to MidiFile.kt