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.Shared/GameObjects/Components/Power/SharedPowerItemCharger.cs

@@ -0,0 +1,21 @@
+using System;
+using Robust.Shared.Serialization;
+
+namespace Content.Shared.GameObjects.Components.Power
+{
+    [Serializable, NetSerializable]
+    public enum CellChargerStatus
+    {
+        Off,
+        Empty,
+        Charging,
+        Charged,
+    }
+
+    [Serializable, NetSerializable]
+    public enum CellVisual
+    {
+        Occupied, // If there's an item in it
+        Light,
+    }
+}