v0.15.0

• Automatically unload history, thus minimizing memory usage for long games
  By default, this happens whenever you commit changes to the database.
• Improved support for ELiDE widgets in games
  • New example sim, awareness.py, to demonstrate this
• Moved game_start into its own module
• Dropped support for Python 3.7 and the lz4 module
• A lot of fixes regarding thread safety and interprocess sync