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/Visualization2D.tsx

@@ -135,6 +135,8 @@ export const Visualization2D: React.FC<
         //this div is to ensure that the size of the visualiser doesnt change when focusings on a subtree
         style={
           {
+            backgroundColor: theme.backgroundWash,
+            borderRadius: theme.borderRadius,
             overflowY: 'auto',
             overflowX: 'hidden',
             position: 'relative', //this is for the absolutely positioned overlays