Use reusable OOCSS for the filter bar on the Add Themes page

[paulwilde]

The attached patch refactors .theme-navigation to use a generic .wp-filter for all CSS selectors/JS hooks on the Add Themes page to increase reusability such as for use with the media grid in 4.0 rather than having to duplicate the styling.

I've also added a reusable .wp-title-count inside common.css which shows a count of themes used on wp-admin/themes.php.

[helen]

In 29217:

Use more generic, reusable CSS for the filter bar on Add Themes. This will allow us to reuse the UI in other places, such as the Media Library. props paulwilde. fixes #28794.

[paulwilde]

drawer-grouping-2.diff:

• Renames 'wp-filter-group-wide' to 'wp-filter-group-2-5' to be more understandable as a grid column. Also added 1-5, 3-5 and 4-5.
• Utilises calc to ensure that the groups have consistent spacing rather than having a bigger gap on the right side.