Improve marker visualization for mapping.

Command renamed to showmarkers.
Actually permanent (so if you place markers with it active, you immediately see them).

Content.Client/GameObjects/Components/Markers/MarkerComponent.cs

@@ -0,0 +1,30 @@
+using Content.Client.GameObjects.EntitySystems;
+using Robust.Client.Interfaces.GameObjects.Components;
+using Robust.Shared.GameObjects;
+using Robust.Shared.GameObjects.Systems;
+
+namespace Content.Client.GameObjects.Components.Markers
+{
+    [RegisterComponent]
+    public sealed class MarkerComponent : Component
+    {
+        public override string Name => "Marker";
+
+        protected override void Startup()
+        {
+            base.Startup();
+
+            UpdateVisibility();
+        }
+
+        public void UpdateVisibility()
+        {
+            var system = EntitySystem.Get<MarkerSystem>();
+
+            if (Owner.TryGetComponent(out ISpriteComponent sprite))
+            {
+                sprite.Visible = system.MarkersVisible;
+            }
+        }
+    }
+}

Content.Client/GameObjects/EntitySystems/MarkerSystem.cs

@@ -0,0 +1,36 @@
+using Content.Client.GameObjects.Components.Markers;
+using Robust.Shared.GameObjects;
+using Robust.Shared.GameObjects.Systems;
+
+namespace Content.Client.GameObjects.EntitySystems
+{
+    public sealed class MarkerSystem : EntitySystem
+    {
+        private bool _markersVisible;
+
+        public override void Initialize()
+        {
+            base.Initialize();
+
+            EntityQuery = new TypeEntityQuery<MarkerComponent>();
+        }
+
+        public bool MarkersVisible
+        {
+            get => _markersVisible;
+            set
+            {
+                _markersVisible = value;
+                UpdateMarkers();
+            }
+        }
+
+        private void UpdateMarkers()
+        {
+            foreach (var entity in RelevantEntities)
+            {
+                entity.GetComponent<MarkerComponent>().UpdateVisibility();
+            }
+        }
+    }
+}