Dequeue accordion.js and copy logic into customize-controls.js
Integrate accordion logic into Section model
Abstract common code between Panel and Section into Container base class
Factor out panel.sections() and section.controls() into _children() base class method
Fix up bubbleChildValueChanges
Integrate Panel accordion logic into model; add behavior for meta accordion sections
Merge pull request #35 from xwpco/trac-28709-accordion

[WIP] #28709: Integrate accordion.js logic into customize.Panel and customize.Section models