[dialog] split out modal options

The modal option for dialogs should only be a boolean. Setting the css for the overlay should be it's own option.

$('#foo').dialog({
    modal: {
        opacity: 0.5
    }
});

becomes:

$('#foo').dialog({
    modal: true,
    overlay: {
        opacity: 0.5
    }
});

This will allow the user to set a default configuration for all modal dialogs when #2295 is complete.