Customizer: Extract content markup for panels to its own method, `WP_Customize_Panel::render_content()`.

This allows to override the behavior of a panel, or even to completely replace its contents with something other than controls or sections.

props celloexpressions.
fixes #29324.

git-svn-id: https://develop.svn.wordpress.org/trunk@29950 602fd350-edb4-49c9-b593-d223f7449a82