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/tree/Tree.tsx

@@ -213,6 +213,7 @@ export function Tree2({
           //We use this normal divs flexbox sizing to measure how much vertical space we need for the child div
           ref={grandParentRef}
           style={{
+            paddingLeft: theme.space.medium,
             height: '100%',
             width: '100%',
           }}>