GridColumnPicker mult-column support selectors

GridColumnPicker mult-column support selectors

Adds a number spinner to each sortable column in the grid column picker.
A Sort Priority of '0' means no sorting is applied, a priority of 1 is
highest priority, 2 is second, etc.

Adds support for an onSortchange handler, which is passed the list of
fields to sort in order.

Adds support for an onLoad handler, which is called after the initial
retrieval of the user persist settings.  This is important for the
initial grid load, which may need to wait until after the sort columns
are known (to avoid re-fetching on the initial load).

Signed-off-by: Bill Erickson <berick@esilibrary.com>