Self-Contained mode support??

[MakovWait]

https://docs.godotengine.org/en/latest/tutorials/io/data_paths.html#self-contained-mode

1. Add a checkbox to add/remove ._sc_ or _sc_ in the same folder to toggle self-contained mode.
2. ~~Add an indicator whether editor is self-contained or not (check the ._sc_ _sc_ file in the same dir) and display it in the way we render project features.~~ done

I am not sure we need to support the first option. For now, it is sufficient to implement the second one.