title: How to add custom columns to grid items designer in FireMonkey
body: <p>I have created a custom column <code>TComboColumn = class(FMX.Grid.TColumn)</code> for <code>TGrid</code>. Currently I have to add this column to grids in the code, but I would like to be able to do this using the <code>TGrid</code> items designer, like I can when adding default column types (<code>TColumn</code>, <code>TStringColumn</code>, <code>TCheckColumn</code> etc).</p>