GizmoUtilities includes UnityEditor

[SonicZentropy]

Was just helping a friend solve his Unity build errors and they came from this package. GizmoUtilities.cs attempts to reference UnityEditor.dll, which is perfectly fine in-editor, but is stripped from Builds. UnityEditor.dll does not exist during or after build, so code that isn't in an Editor/ folder cannot use it. This means it's currently impossible to build this package as-is within Unity. The portions of GizmoUtilities dealing with Handle need to be moved to an Editor/ folder instead.