This layout implements the column arrangement for Ext.form.CheckboxGroup and Ext.form.RadioGroup.
It groups the component's sub-items into columns based on the component's
columns and Ext.form.CheckboxGroup.vertical config properties.
By default, CheckboxGroup allocates all available space to the configured columns meaning that
column are evenly spaced across the container.
To have each column only be wide enough to fit the container Checkboxes (or Radios), set autoFlex to false
Defaults to: true
Iterates over all passed items, ensuring they are rendered. If the items are already rendered,
also determines if the items are in the proper place in the dom.