Inventory Input (#503)

* Create ItemSlotButton

* Refactor inventory buttons

Refactor so that KeyBind handling of inventory and hands are the same.

* Refactor InventoryInterfaceController to call ItemSlotManager with entities

This change allows HandsGUI and InventoryInterfaceController to just call ItemSlotManager.OnButtonPressed with an entity instead of a slot.

* Add CooldownCircle to ItemSlotButton

This allows Hands and Inventory to have cooldown circles on their ItemSlots.

* Refactor HandsGUI to use ItemSlots

This allows functionality and GUI to be the same between Inventory and Hands.

Added clicking empty hand to switch ActiveHand to clicked hand.

* Implement CooldownCircle in ItemSlotManager

Reorganize files

Content.Client/UserInterface/IItemSlotManager.cs

@@ -0,0 +1,13 @@
+using Content.Client.GameObjects;
+using Robust.Client.UserInterface;
+using Robust.Shared.Interfaces.GameObjects;
+
+namespace Content.Client.UserInterface
+{
+    public interface IItemSlotManager
+    {
+        void Initialize();
+        bool OnButtonPressed(GUIBoundKeyEventArgs args, IEntity item);
+        void UpdateCooldown(ItemSlotButton cooldownTexture, IEntity entity);
+    }
+}