…b.com/knishiura-lab/wp-widget-customizer into develop

Closes #72

Props @MichaelArestad

* Adds Reorder link next to Add Widget link
* Reorder link causes reorder buttons with each widget to appear
* Widgets can be moved up/down via buttons
* Widgets can be moved to other sidebars via new subpanel (not yet fully functional)

Other changes:
* Use Backbone for keeping track of which sidebars are rendered
* Add tabIndex to new UI elements for keyboard accessibility
* Add methods for obtaining a widget’s sidebar control,
  and a sidebar’s widget controls
* Add widget control methods for moving a widget up/down,
  and for getting its position in the sidebar.

…other sidebar

* scope widget element lookups by associated sidebar
* instead of appending to the parent, we should append relative to the first widget found

* Remove required preview refresh when widget is moved to another sidebar
* Ensure that widget moved to other sidebar without drag/drop isn’t trashed

Cool.

Add more icon matchers, clean up CSS, improve focus states

* Send all sidebars_widgets settings, not just the setting for the widget being edited
* Let incoming customized POST data completely override sidebars_widgets settings; stop merging
* Stop deleting widget instance settings from preview customizer because unnecessary and
  because of possible race condition when moving widgets across sidebars

Focus on first focusable instead of on first visible input

Add keyboard-accessible means of reordering widgets