Add a <T:Any>.addTo(GUI) extension method to ease adding items to the sidebar

orx-gui/src/main/kotlin/Gui.kt

@@ -550,4 +550,10 @@ class GUI : Extension {
             updateControls()
         }
     }
+}
+
+@JvmName("addToGui")
+fun <T : Any> T.addTo(gui: GUI): T {
+    gui.add(this)
+    return this
 }