Add power cell and weapon chargers (#430)

* Add power cell and weapon chargers

Functionality's similar to SS13. The cell charger won't show the discrete charging levels because effort to not make it spam appearance updates over the network.
There's some stuff I wasn't sure on:
1. Updating power? Particularly around fixing rounding
2. Duplicating the full and empty sprites as I couldn't figure out a way to not have AppearanceVisualizer throw if the state isn't found
3. I made a BaseCharger abstract class under the assumption that when a mech / borg charger is added it would also inherit from it.
4. GetText currently isn't localized

* Address PJB's feedback

Updated the BaseCharger
* Change nullref exception to invalidoperation
* If the user doesn't have hands it will just return instead

Content.Client/GameObjects/Components/Power/PowerChargerVisualizer2D.cs

@@ -0,0 +1,77 @@
+using Content.Shared.GameObjects.Components.Power;
+using JetBrains.Annotations;
+using Robust.Client.GameObjects;
+using Robust.Client.Interfaces.GameObjects.Components;
+using Robust.Shared.Interfaces.GameObjects;
+
+namespace Content.Client.GameObjects.Components.Power
+{
+    [UsedImplicitly]
+    public class PowerChargerVisualizer2D : AppearanceVisualizer
+    {
+        public override void InitializeEntity(IEntity entity)
+        {
+            base.InitializeEntity(entity);
+
+            var sprite = entity.GetComponent<ISpriteComponent>();
+
+            // Base item
+            sprite.LayerMapSet(Layers.Base, sprite.AddLayerState("empty"));
+
+            // Light
+            sprite.LayerMapSet(Layers.Light, sprite.AddLayerState("light-off"));
+            sprite.LayerSetShader(Layers.Light, "unshaded");
+        }
+
+        public override void OnChangeData(AppearanceComponent component)
+        {
+            base.OnChangeData(component);
+
+            var sprite = component.Owner.GetComponent<ISpriteComponent>();
+
+            // Update base item
+            if (component.TryGetData(CellVisual.Occupied, out bool occupied))
+            {
+                // TODO: don't throw if it doesn't have a full state
+                sprite.LayerSetState(Layers.Base, occupied ? "full" : "empty");
+            }
+            else
+            {
+                sprite.LayerSetState(Layers.Base, "empty");
+            }
+
+            // Update lighting
+            if (component.TryGetData(CellVisual.Light, out CellChargerStatus status))
+            {
+                switch (status)
+                {
+                    case CellChargerStatus.Off:
+                        sprite.LayerSetState(Layers.Light, "light-off");
+                        break;
+                    case CellChargerStatus.Empty:
+                        sprite.LayerSetState(Layers.Light, "light-empty");
+                        break;
+                    case CellChargerStatus.Charging:
+                        sprite.LayerSetState(Layers.Light, "light-charging");
+                        break;
+                    case CellChargerStatus.Charged:
+                        sprite.LayerSetState(Layers.Light, "light-charged");
+                        break;
+                    default:
+                        sprite.LayerSetState(Layers.Light, "light-off");
+                        break;
+                }
+            }
+            else
+            {
+                sprite.LayerSetState(Layers.Light, "light-off");
+            }
+        }
+
+        enum Layers
+        {
+            Base,
+            Light,
+        }
+    }
+}