hierachy and parent child system updates

- started working on an editor hierarchy window
- testing filters
- testing parent child relations without system
- testing root component
- thinking about how to get all entities that are not a child, but also don't have an other identifying component

Nerfed.Editor/Systems/EditorHierarchyWindow.cs

@@ -0,0 +1,60 @@
+using ImGuiNET;
+using MoonTools.ECS;
+using Nerfed.Runtime.Components;
+
+namespace Nerfed.Editor.Systems
+{
+    internal class EditorHierarchyWindow : MoonTools.ECS.DebugSystem
+    {
+        private readonly Filter rootEntitiesWithTransformFilter;
+        private readonly Filter rootEntitiesFilterBroken;
+        private readonly Filter rootEntitiesFilter;
+
+        public EditorHierarchyWindow(World world) : base(world)
+        {
+            rootEntitiesWithTransformFilter = FilterBuilder.Include<LocalTransform>().Exclude<Child>().Build();
+
+            // TODO: this doesn't work.
+            rootEntitiesFilterBroken = FilterBuilder.Exclude<Child>().Build();
+
+            // Maybe the parent/child functions should add a root component when not being a child.
+            rootEntitiesFilter = FilterBuilder.Include<Root>().Build(); 
+
+            // Maybe instead of a root, if we need a component that is always on an entity and has some use we could create something like a VersionComponent which only hold an int.
+            // The version would update each time something changes on the entity.
+            // 
+        }
+
+        public override void Update(TimeSpan delta)
+        {
+            ImGui.Begin("Hierarchy");
+
+            foreach (Entity entity in rootEntitiesWithTransformFilter.Entities)
+            {
+                DrawEntityAndChildren(entity);
+            }
+
+            ImGui.NewLine();
+
+            foreach (Entity entity in rootEntitiesFilter.Entities)
+            {
+                DrawEntityAndChildren(entity);
+            }
+
+            ImGui.End();
+        }
+        
+        private void DrawEntityAndChildren(in Entity entity, int depth = 0)
+        {
+            string label = new string(' ', depth);
+            ImGui.Text($"{label}{entity.ID} | {GetTag(entity)}");
+
+            depth++;
+            ReverseSpanEnumerator<Entity> childEntities = World.InRelations<ChildParentRelation>(entity);
+            foreach (Entity childEntity in childEntities)
+            {
+                DrawEntityAndChildren(childEntity, depth);
+            }
+        }
+    }
+}