Allow selectors to be used in place of UUIDs/entity strings

Specifically for making NBT data more useful and versatile, being able to use selectors where UUIDs are required (like in NBT data JSON) would be awesome. I can see a reason why this might not yet be a feature: possibly the fact that selectors don't always point to the same one entity from time to time (for example, `@n` would change if another entity moves closer to `@s` than the previous one). I think this could be worked around by having the selectors be automatically substituted for the corresponding entity's UUID when the command is run, meaning that the selectors are just placeholders.

Though that example is pretty specific to one possible problem, I'm sure that kind of thinking could be applied to any other problem.