JS is horribly inefficient on Write -> Post/Link pages when lots of categories

[mdawaffe]

There are probably lots of inefficiencies. This patch addresses two.

1. Whenever a checkbox in "All Categories" is checked/unchecked, it syncs that new checked state with it's partner in "Most Used" and vice versa.
2. On load (and maybe unload in some browsers during some kind of cleanup), wp-lists looks at each category checkbox and makes huge arrays. This is so that the current checked state of all category checkboxes is known (needed for ajax addition of categories).

The attached fixes:

1. By only syncing the categories that are in both lists. To do this, various wp-admin/include/template functions have been tweaked so that they can be aware of what categories are popular.
2. By vastly reducing the number of inputs wp-lists looks at on load and making slight efficiency tweaks to wp-lists.

[ryan]

(In [7669]) More efficient category and list JS from mdawaffe. fixes #6677 for trunk

[ryan]

(In [7670]) More efficient category and list JS from mdawaffe. fixes #6677 for 2.5

[ryan]

Reopening to tack on some more category list improvements.

Attached is some work by mdawaffe that AJAX loads the category list when you click on the "All Categories" tab.

[ryan]

(In [7689]) Demand load the category list. Props mdawaffe. see #6677

[ryan]

This doesn't degrade nicely with JS turned off. We'll try it in trunk and work the kinks out. Not ready for the 2.5 branch.

[Viper007Bond]

Can we make, via a plugin or whatnot, some easy method of keeping the "All Categories" the default? I hate the most-used view and never use it.

[ryan]

Maybe open the last tab used like we do with the editor.

[ryan]

(In [7720]) Revert [7689], gonna try another approach. see #6677

[mdawaffe]

One major improvement that can be made is that the Parent Category select dropdown in the Add New Category section and the list of checkboxes in the All Categories section display the same information, but use different queries/logic so we get no cache benefit.

[ryan]

Calling this fixed. Issues mentioned can be handled in other tickets.