[unity] BoundingBoxFollower `Add Bone Follower` button was always disabled. Also calling BoundingBoxFollower.Initialize() explicitly in editor code now (might resolve some problems with recent Unity versions). Closes #1717.

spine-unity/Assets/Spine/Editor/spine-unity/Editor/Components/BoundingBoxFollowerInspector.cs

@@ -160,6 +160,8 @@ namespace Spine.Unity.Editor {
 
 			}
 
+			if (follower.Slot == null)
+				follower.Initialize(false);
 			bool hasBoneFollower = follower.GetComponent<BoneFollower>() != null;
 			if (!hasBoneFollower) {
 				bool buttonDisabled = follower.Slot == null;