Support row-level control of selectable rows

[mcw0933]

This pull request:

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.

So, for example:

multiselect: true,
multiselectField: "active",
showSelectAll: true

would give you a grid where only the rows with their "active" column value being truthy are selectable. Clicking select all will only select those rows, or they can be individually selected. The beforeSelectRow event will be fired for each of those rows.