[unity] Move asset inspector scripts to the main Editor folder.

spine-unity/Assets/spine-unity/Asset Types/Editor.meta

@@ -1,9 +0,0 @@
-fileFormatVersion: 2
-guid: cbe5d97ed1d75964cab2e2882a52a200
-folderAsset: yes
-timeCreated: 1455489536
-licenseType: Free
-DefaultImporter:
-  userData: 
-  assetBundleName: 
-  assetBundleVariant: 

spine-unity/Assets/spine-unity/Editor/AtlasAssetInspector.cs

@@ -126,8 +126,8 @@ namespace Spine.Unity.Editor {
 
 			for (int i = 0; i < materials.arraySize; i++) {
 				SerializedProperty prop = materials.GetArrayElementAtIndex(i);
-				Material mat = (Material)prop.objectReferenceValue;
-				if (mat == null) {
+				var material = (Material)prop.objectReferenceValue;
+				if (material == null) {
 					EditorGUILayout.HelpBox("Materials cannot be null.", MessageType.Error);
 					return;
 				}