v0.19.0 small files

This release changes the default storage backend from SQLite to ParquetDB, which takes something like 1/20 as much space for the same data.

I did a lot of refactoring, too, and improved test coverage. Many mysterious crashes to do with database integrity and interprocess synchronization don't happen anymore.

Other changes in the Lisien core:

• The engine's `time` property now includes the tick, as well as the branch and turn.
• Added the ability to record "replays" of Elide sessions. You won't really be able to play them back, exactly. They run the same commands on the Lisien core that Elide would. If you set a replayfile in elide.ini's [lisien] section, then send the file to me, I'll be able to reproduce whatever bug you ran into.
• When using the proxy interface, you can register functions to be called whenever the time changes, as `engine_proxy.time.connect(function)`; or whenever the rules engine has simulated a turn, as `engine_proxy.next_turn.connect(function)`. Also available in the core proper.
• Filled in some missing parts of the proxy API.
• Fixed inconsistent handling of planned changes (in `with engine.plan():` blocks).
• Fixed Thing objects sometimes hanging around after deletion, depending on when you closed the engine.
• The old allegedb submodule no longer exists. Its functionality is now part of Lisien proper.

Changes in the Elide frontend interface:

• Fixed stats running off the left of the window
• Fixed the grid view not being visible.
• Fixed configuration of the stat widgets.