title: Trigger event when headerCheckboxSelection is marked?
body: <p>I've been reading the <code>ag-grid</code> documentation and I haven't been able to find out if there's any event to know when the checkbox to select or un-select all column was clicked.</p>
  
  <p>I know I can use <code>onRowSelected()</code> but I need to know when <em>Select</em> or <em>un-Select All</em> is clicked so I can make a different operation.</p>