Simplify usage of attributes.

[MaximumADHD]

Summary

This commit deprecates using the Attributes property in $properties, in favor of a direct $attributes field in the JSON instances (or attributes in *.model/meta.json files). Unambiguous value types like string, boolean, and number can be written as fields of $attributes directly.

The Problem

In trying to clean up the existing structure for writing attributes (i.e. adding unambiguous value types), it became apparent the following case would cause problems with how rojo handles explicit types:

{
  "$properties": {
    "Attributes": {
      "String": "Test"
    }
  }
}

Rojo interprets the value of this object's attributes to be an explicit string "Test", rather than a single attribute named "String" with "Test" as its value. Working around this requires the value of test to be encapsulated as so:

{
  "$properties": {
    "Attributes": {
      "String": {
        "String": "Test"
      }
    }
  }
}

The solution is to explicitly make $attributes a separate field:

{
  "$attributes": {
    "String": "Test"
  }
}

This not only looks cleaner, but it also clears up any ambiguity since $attributes is explicitly HashMap<String, Variant>.

Compatibility

This change is forward compatible with the existing use of the "Attributes" property.
It just requires noisy explicit typing to work as expected.