Adds a UI for gas mixers (#5165)

Co-authored-by: E F R <602406+Efruit@users.noreply.github.com>
Co-authored-by: Paul Ritter <ritter.paul1@googlemail.com>
Co-authored-by: ike709 <ike709@github.com>

Resources/Prototypes/Entities/Structures/Piping/Atmospherics/trinary.yml

@@ -76,6 +76,10 @@
         - type: SubFloorShowLayerVisualizer
         - type: PipeConnectorVisualizer
         - type: PipeColorVisualizer
+    - type: UserInterface
+      interfaces:
+        - key: enum.GasMixerUiKey.Key
+          type: GasMixerBoundUserInterface
     - type: GasMixer
       inletOne: inlet
       inletTwo: filter