UXCore 4.0.0-pre.3 doesn't build

> @keveleigh > 
> I downloaded the UXCore 4.0.0-pre.3 and can't build anymore due to this use of editor only functions. This script is not in the "org.mixedrealitytoolkit.uxcore/Editor" folder. 

 _Originally posted by @anonymous2585 in [db1cbd6](https://github.com/MixedRealityToolkit/MixedRealityToolkit-Unity/commit/db1cbd69e7a33486ac1b43f059c3b5bb3b5356bd#r188251979)_