Facilitating embedding customizer controls outside of sections

[westonruter]

Controls are currently assumed to be always contained within sections in the customizer. This makes it difficult to reuse the controls in other contexts, such as embedding multiple controls inside another control or embedding a control outside the context of the customizer entirely. This will facilitate embedding customize controls on the frontend for contextual editing without having to have the customizer sidebar open or even having to go into customize.php at all.

Some of the hacks required to get controls to appear outside of the customizer can be seen in ​https://github.com/xwp/standalone-customizer-controls

The Media control in particular needs to be updated to remove the logic resize the player controls when the section is expanded. The embed method (used by widgets) also needs to not wait generally for a contained section to expand.

Key dependency for #29071 (Make it easier to include an instance of the Customizer outside of customize.php)

Depends on or is closely related to #37964 (Allow customizer controls to be encapsulated by accepting pre-instantiated settings)

[celloexpressions]

This is low priority for 4.7, but will be needed eventually and should switch back to normal priority if we punt it.

[chriscct7]

Punting per bugscrub discussion