Missing capability to override Walker_Cat*

[AaronCampbell]

Allow #8673

Summery: PATCH to add the ability to override Walker_Category, Walker_CategoryDropdown, and Walker_Category_Checklist.

The Comment Walker can be overridden by a user supplied object, and #8673 added that capability to the page walker (Walker_Page) and the dropdown page walker (Walker_PageDropdown).

Attached is a patch which adds the ability to supply your own walker objects for the category walkers. It uses the same method as for Walker_Comment (a instantiated class supplied to the wp_list_pages() or wp_dropdown_pages() function call. I added a check to make sure the walker that's passed in is of the Walker class or has it as one of its parents. That check isn't in other areas of WP, so if you want me to remove it I can.

(In [10245]) Allow overriding cat walker. Props AaronCampbell. fixes #8682 for 2.7.

(In [10246]) Allow overriding cat walker. Props AaronCampbell. fixes #8682 for trunk