SuperTiled2Unity Version 1.5.4

Improvements with SuperTiled2Unity version 1.5.4

• BUG FIX: Unity package install no longer overwrites changes made to `ST2U Settings`
• NEW FEATURE: Can use `unity:sortingOrder` custom property to override sorting order in layer for a tilemap or object.
• BUG FIX: Compiles again in 2018.3.x versions of Unity
• BUG FIX: Fixed crash when re-importing Tiled maps in 2018.3.x versions of Unity