Customize: Fix deep-linking to sections for themes via `autofocus[section]` query parameter.

* Expand containing panel when expanding themes section.
* Consolidate UI changes related to a section's expanded state change.
* Prevent collapsing current section when expanding.
* Auto-expand first themes section when expanding panel if one is not expanded already.

See #37661, #42354.
Fixes #42360.

Built from https://develop.svn.wordpress.org/trunk@42033


git-svn-id: https://core.svn.wordpress.org/trunk@41867 1a063a9b-81f0-0310-95a4-ce76da25c4cd