Add support for checkbox columns to the datagrid.
This introduces CheckboxColumn, which can be used by a datagrid to

handle multiple selection of entries. It will display a checkbox for

every row, as well as a checkbox in the header that, when clicked, will

toggle selection of all the checkboxes in each row.
This is based heavily on on Graeme Coupar's work on

https://reviews.reviewboard.org/r/3796/

Tested with the accompanying change for Review Board. Tested:

Toggling individual checkboxes.
Toggling the checkbox in the header.
Adding/removing the column.