Adds row-level control for row multiselect

Adds a "multiselectField" property which, when set, conditionally enables/disables selectability of that row based on the field's value.  Default is false (all rows are selectable).

Adds a "showSelectAll" property to toggle display the select all rows checkbox.  Default is true (show it).

Triggers the beforeSelectRow event, per row, during a select all click.