Add materials expression

Add 'Materials' expression, which will return a JSON string with the instance's current material names and paths (if the material was loaded during runtime from a file or URL.)

Example return string:

[{"name":"Cleric_Texture","path":"Cleric_Texture"},{"name":"Cleric_Staff_Texture","path":"Cleric_Staff_Texture"},{"name":"baseColor","path":"baseColor.png"},{"name":"checker","path":"checker.png"}]

This format is useful as input to the C3 JSON Object for use in events, etc.