fix: Allow for saving inside prefabs, fixes #73

[GotoFinal]

Based on https://stackoverflow.com/questions/65001961/unity3d-how-to-save-part-of-a-prefab-instance-as-a-prefab
Might require more testing to make sure it always behaves as expected. I tested on my prefab setup, where I removed the emoface prefab from the actual prefab and scene and then:

1. Used Apply To Avatar on this clean prefab instance - worked like before
2. Used Apply to Avatar again on this instance - worked like before overriding the old object.
3. Saved that instance as an actual prefab
4. Used Apply to Avatar again on this instance - it saved the changes directly to actual prefab, updating all other variants using this prefab. (this crashed before)

The solution might seems a bit hacky I guess... But personally it does what I would like the tool to do.

[suzuryg]

Thanks for the suggestion!
Actually, there is an implementation for #73 that I had been working on locally, but I had not pushed it to github...
I have just pushed it and created a PR (#106) , so I would like to close this PR.
I may make some code rework on this one before merging.