Add track controls

Summary: Project: https://docs.google.com/document/d/1x-1ShlxXCM2od9JMO6PP14Cuqq0OrVe1Qd7FUwTos9A/edit#heading=h.fiq3qxllvkl

Reviewed By: ivanmisuno

Differential Revision: D44306047

fbshipit-source-id: 393741f9255dea3d582ac43a4733aa835f5d4a27

desktop/flipper-plugin/src/ui/Layout.tsx

@@ -72,6 +72,7 @@ export type SplitLayoutProps = {
   gap?: Spacing;
   children: [React.ReactNode, React.ReactNode];
   style?: CSSProperties;
+  className?: string;
 } & SplitHorizontalResizableProps &
   SplitVerticalResizableProps;