Redesign Split Tree and visualiser into panels

Summary: This looks a lot neater and allows us to have separate dedicated visualiser header

Reviewed By: mweststrate

Differential Revision: D47626867

fbshipit-source-id: 843721853e0ff88837afbb9bf3f510a908160d12

desktop/plugins/public/ui-debugger/components/Controls.tsx

@@ -44,7 +44,7 @@ export const Controls: React.FC = () => {
     useState(false);
 
   return (
-    <Layout.Horizontal pad="small" gap="small">
+    <Layout.Horizontal gap="medium" pad="medium">
       <Input
         value={searchTerm}
         onChange={(e) => {