Ability to pass arguments to callback for dashboard widget

[jdgrimes]

There are times where it would be useful to be able to pass some arguments to the callback function for a dashboard widget when adding it with wp_add_dashboard_widget(). This would make it possible to have multiple 'instances' of a dashboard widget if desired.

For reference, I'm not the only one who has felt this need: ​http://wordpress.org/support/topic/add-parameters-to-the-callback-function-in-wp_add_dashboard_widget?replies=2

Also, add_meta_box(), which wp_add_dashboard_widget() calls, already supports this. All we need to do is add another argument to wp_add_dashboard_widget() and pass it along to add_meta_box(). The attached patch does just that.

[jdgrimes]

Adds $callback_args parameter to wp_add_dashboard_widget()

[nacin]

In 25454:

Allow callback args to be passed from wp_add_dashboard_widget() to add_meta_box().

props jdgrimes.
fixes #25152.