Disable test AgentDetachesAndAttachesToSurface because the required functionality is missing altogether.

(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.

Inhibit recording Undo history when baking or clearing a NavMeshSurface (at the time when the old NavMeshData is actually replaced).
No need to record prefab instance property modifications when we're using a serialized object for modifying the NavMeshData.

Clear prefab stage dirtiness in tests where prefab changes need to be discarded.