make cell template the content of DataGridTemplateColumn

saves a lot of bloat when decalring TemplatedColumns.

src/Avalonia.Controls.DataGrid/DataGridTemplateColumn.cs

@@ -22,6 +22,7 @@ namespace Avalonia.Controls
                 o => o.CellTemplate,
                 (o, v) => o.CellTemplate = v);
 
+        [Content]
         public IDataTemplate CellTemplate
         {
             get { return _cellTemplate; }