feat: show dialog after changing InstanceHandling to warn against los…

…t changes (#729)

* Adding dialog to warn user that they may lose changes.

* Giving user the option to never show the warning again.

* Save setting per session, not machine

Editor/Scripts/MeshSyncServerInspector.cs

@@ -7,6 +7,7 @@ namespace Unity.MeshSync.Editor  {
 [CustomEditor(typeof(MeshSyncServer))]
 [InitializeOnLoad]
 internal class MeshSyncServerInspector : BaseMeshSyncInspector {
+    private const string OPT_OUT_INSTANCE_HANDLING = "MeshSync.InstanceHandling.OptOut";
 
 //----------------------------------------------------------------------------------------------------------------------
 
@@ -87,14 +88,20 @@ public void DrawServerSettings(MeshSyncServer t)
         }
     }
 
-    private void DrawInstanceSettings(MeshSyncServer t)
-    {
+    private void DrawInstanceSettings(MeshSyncServer t) {
         var style = EditorStyles.foldout;
-        style.fontStyle = FontStyle.Bold;
+        style.fontStyle        = FontStyle.Bold;
         t.foldInstanceSettings = EditorGUILayout.Foldout(t.foldInstanceSettings, "Instances", true, style);
-        if (t.foldInstanceSettings)
-        {
-            t.InstanceHandling = (BaseMeshSync.InstanceHandlingType)EditorGUILayout.EnumPopup("Instance handling", t.InstanceHandling);
+        if (t.foldInstanceSettings) {
+            var newInstanceHandling =
+                (BaseMeshSync.InstanceHandlingType)EditorGUILayout.EnumPopup("Instance handling", t.InstanceHandling);
+
+            if (t.InstanceHandling != newInstanceHandling &&
+                EditorUtility.DisplayDialog("Warning",
+                    "Changing the instance handling mode will delete any prefabs and previously synced objects for this server. Are you sure you want to do this?",
+                    "Yes", "No", DialogOptOutDecisionType.ForThisSession, OPT_OUT_INSTANCE_HANDLING)) {
+                t.InstanceHandling = newInstanceHandling;
+            }
 
             DrawPrefabListElement(t);
         }