UI: Rename blending mode "Additive" to "Add" in UI

UI/data/locale/en-US.ini

@@ -535,7 +535,7 @@ BlendingMethod.SrgbOff="SRGB Off"
 # blending modes
 BlendingMode="Blending Mode"
 BlendingMode.Normal="Normal"
-BlendingMode.Additive="Additive"
+BlendingMode.Additive="Add"
 BlendingMode.Subtract="Subtract"
 BlendingMode.Screen="Screen"
 BlendingMode.Multiply="Multiply"