Make sidebar buttons dynamic to allow for 'hot-swappable' pages/areas

[taylortom]

For example, I want to be able to unplug the existing theme picker, and swap with a theme editor while allowing for the picker to be re-added/swapped back in later.

Currently, the editor sidebar 'areas' are very much hard-coded in the part_editorCommon.hbs template, sidebarItemView.js and editor.js (possibly also elsewhere).