Not all references to wp-admin.css get cache busted

[mdawaffe]

Most everywhere, wp-admin.css is cache busted by appending a version parameter to the query string.

But not everywhere.

Additionally, we don't include rtl stylesheets in all places.

Attached:

1. wp_admin_css_uri( $file = 'wp-admin' ) returns the absolute URL of the requested admin css file. $file is the name of the file (with path relative to /wp-admin) and without the .css extension.
2. wp_admin_css( $file = 'wp-admin' ) echoes <link rel="stylesheet" /> markup for the requested admin css file and it's rtl equivalent if necessary.

Complete with filters.

Or we could just go through and make sure every css file is cache busted appropriately.

[ryan]

(In [5965]) Make css cache busting and rtl inclusion uniform across admin. Props mdawaffe. fixes #4830