WAIQWatch plugin improvements

Summary:
Improving usability of the WAIQWatch plugin.
Also fixing a bug in `MasterDetail`, which didn't use `sidebarSize` as `height` when detail panel is shown at the bottom.
iOS companion diff here: D28756787

Reviewed By: tomw

Differential Revision: D28751609

fbshipit-source-id: ba27ae7ce3596179752c963dba1bf4b994767c2a

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

@@ -238,7 +238,7 @@ export function MasterDetail<T extends object>({
       );
     case 'bottom':
       return (
-        <Layout.Bottom resizable width={sidebarSize}>
+        <Layout.Bottom resizable height={sidebarSize}>
           {table}
           {sidebar}
         </Layout.Bottom>