title: Accessing ag-grid column visibility event completion
body: <p>we are using ag-grid (v17.1) with angular in our project. We (need to display) displaying 160+ at a time in grid. On Select All / Deselect All, in tool panel, it takes around 15-20 seconds to show/hide the columns. I wanted to show overlay spinner for such event. Overriding the column visibility event does not provide us the exact time, when the ag-grid has completed with rendering the columns. I wanted clarification on below questions.</p>
  
  <ol>
  <li>How can we identify if ag-grid has completed rendering columns?</li>
  <li>Can we override tool panel and customize it? (I checked on ag-grid documentation and could not find a way to override tool panel)</li>
  </ol>