Update the project to Unity 2019.3.0f6 and remove the deprecated GUI Layout component from the main camera objects in the example scenes.

Disable test AgentDetachesAndAttachesToSurface because the required functionality is missing altogether.

Updated scripts to call PrefabUtility.GetCorrespondingObjectFromSource() instead of the deprecated PrefabUtility.GetPrefabParent().

(cherry picked from commit bcdc1450)

Fixed the use of internal methods for disabling the NavMeshData object field in the custom inspector.

Used ExecuteAlways attribute to make sure that NavMeshSurfaces from prefabs can be inspected during playmode.

Filter out modifier volumes that are not in the same stage as the NavMeshSurface that bakes the data.

Added tests for verifying that NavMeshModifierVolume affects only NavMeshes baked in the stage to which the modifier belongs.