Sandbox window improvements (#578)

* Several sandbox manager improvements

- Bound sandbox manager to B key as it lists on the UI
- Bound entity spawner to F5
- Bound tile spawner to F6
- Made entity spawner and tile spawner toggle instead of repeated spawning new windows from the sandbox panel

* Move relevant keyfunctions from engine to content

Turns out it was unnecessary to have the key functions in the engine as all code using the ones added here are in content.

Content.Shared/Input/ContentKeyFunctions.cs

@@ -21,5 +21,8 @@ namespace Content.Shared.Input
         public static readonly BoundKeyFunction ThrowItemInHand = "ThrowItemInHand";
         public static readonly BoundKeyFunction ToggleCombatMode = "ToggleCombatMode";
         public static readonly BoundKeyFunction MouseMiddle = "MouseMiddle";
+        public static readonly BoundKeyFunction OpenEntitySpawnWindow = "OpenEntitySpawnWindow";
+        public static readonly BoundKeyFunction OpenSandboxWindow = "OpenSandboxWindow";
+        public static readonly BoundKeyFunction OpenTileSpawnWindow = "OpenTileSpawnWindow";
     }
 }