v0.20.0 Android support

This release adds support for Android. Currently, the Android version is only distributed on Itch, not on the Play Store.

Elide now has a main menu. The "export game" option simply compresses a game's folder into a zip file, and "import game" unzips it; they're needed for sharing games on Android, where the user can't reasonably access Elide's files directly, but are not very useful on desktop.

Lisien and Elide now support Python 3.13, too, though I'm sticking with 3.12 for the Windows build until Kivy starts distributing binary wheels for Python 3.13 on Windows.

Some major crashes were fixed:

• When resuming a game that was closed at the very beginning of a branch
• Intermittently when deleting entities, if an automatic keyframe snap happened to occur mid-deletion. Fixed by making deletion atomic.