v0.20.2

This release cleans up a lot of internals, ruling out some types of data loss.

• Overhauled the internal WindowDict class and its relatives, ruling out a bunch of race conditions that caused intermittent crashes
• Made deltas behave more consistently, ruling out some types of data loss
• Overhauled the tracking of units and users
• Made entity deletion really atomic, this time
• Fixed keyframe snaps sometimes happening after a change had been recorded to the database, but before it had been cached, resulting in the loss of a single change to the game state
• Fixed deleting keyframes