Allow sorting / grouping of "allowed views" in field settings page

We have one site with a ton of views. When listed unsorted, it becomes very tedious to ensure all the correct view groups have been selected. We'd like to select them from an alphabetically sorted list, grouped by tags.

The attached patch allows for the checkboxes to be listed in one of three ways, depending on the viewfield_options_sort setting:

1. Alphabetically, grouped by tag
2. Alphabetically, no grouping
3. As returned by views_get_all_views() (current behavior)