Support nesting Parent

[ewd3v]

This pull request adds the support to do nested Parent property writes and closes #47 , like this:

local frame: Frame = ...

create "TextLabel" {
    { Parent = frame }
}

This makes Parent property writes more consistent with other properties.

I also added tests to check for this as how it's supported is a little different from how other nested properties are implememented.
(I also included a fix for mock instances as the .Parent property was not a proxy when reading it, please review.)

I'm not sure the way I added this is the best way but it does work (I'm using the Cache object to store what instance it should be parented to).

This pr also makes Parent property get checked for duplicates at the same depth like other properties (was an easy fix and I'm assuming it's intended anyways, if not tell me and I'll remove this behaviour from the pr.)

[centau]

Thanks