Improve the categories widget

[rob1n]

Extend the categories widget to use all of wp_list_categories()'s options.

Namely:

• Customizable sort column
• Hierarchical display
• Toggle showing category counts
• Toggle showing empty categories
• Toggle using description for title
• Add showing categories which are child of a certain category
• Toggle showing feed
• Exclude comma-separated categories
• Multiple instances

For the feed stuff, instead of allowing arbitrary feed images, always use our included rss.png. That image is well known now. Perhaps we should collapse all of the feed options into a "Show Feeds" option that simply controls whether the rss.png image (linked to the category feed) is shown next to each category.

Makes sense.

adds support for multiple instances

(In [5669]) Add support for multiple instances of the category widget. see #4285

(In [5673]) Some more multiple category widget stuff. see #4285

(In [5675]) Revert multiple category stuff. see #4285

Changing the category widget's options array breaks existing category widgets. Some upgrade code is needed.

Got it fixed on wpcom. I'll merge it over.

(In [5717]) Upgrade code for multiple category widgets. Props Andy. see #4285

See #9780

closing this as dup of #9780, as all of the remaining feature requests could then be done using a plugin.

#22729 was marked as a duplicate.