(core) Adding UI for timing API

Summary:
Adding new buttons to control the `timing` API and a way to view the results
using virtual table features.

Test Plan: Added new

Reviewers: georgegevoian

Reviewed By: georgegevoian

Subscribers: paulfitz

Differential Revision: https://phab.getgrist.com/D4252

app/client/ui2018/modals.ts

@@ -672,7 +672,7 @@ const cssModalBacker = styled('div', `
   }
 `);
 
-const cssSpinner = styled('div', `
+export const cssSpinner = styled('div', `
   display: flex;
   align-items: center;
   height: 80px;