Fix xcassets icon generation (correct image sizes) + stop overriding user defined assets once generated #801

Job	Run time
Flake8 tests	10s
build_python3_kivy_venv (macos-latest)	11m 11s
build_python3_kivy (macos-latest)	10m 2s
build_python3_kivy_venv (macos-13)	32m 30s
build_python3_kivy (macos-13)	22m 57s
build_updated_recipes (macos-latest)	25s
build_updated_recipes (macos-13)	55s
	1h 18m 10s

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