linux/appimage: generate byte-code files and package them

This makes the app start slightly faster, as byte-code files won't need
to be generated every time users run the app.

https://docs.python.org/3.10/library/compileall.html

appimage/AppImageBuilder.yml

@@ -24,6 +24,8 @@ script:
 
   # Install requirements.
   - python3 -m pip install --ignore-installed --prefix=/usr --root="$TARGET_APPDIR" -r "$SOURCE_DIR/../requirements.txt" certifi
+  # Generate byte-code files beforehand, for slightly faster app startup.
+  - python3 -m compileall "$TARGET_APPDIR/usr/src/"*.py
 
 AppDir:
   app_info: