Fixes and tweaks for the metabox accordion.

* Fixed hover issues in Customizer.
* Toned down the default active/hover style (as it is normally not the top level menu, and should not be so prominent).
* Allow the active section to be closed.
* Other misc fixes.

Props ocean90, helen. Fixes #23449.

git-svn-id: https://core.svn.wordpress.org/trunk@24734 1a063a9b-81f0-0310-95a4-ce76da25c4cd