Integrate keyboard-accessible widget reordering

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.
Prevent empty widget addition from being flagged as unsanitary
Prevent widget element lookup race condition when moving widget to another sidebar

* scope widget element lookups by associated sidebar
* instead of appending to the parent, we should append relative to the first widget found
Improve logic for cleaning up when widget is removed from sidebar

* Remove required preview refresh when widget is moved to another sidebar
* Ensure that widget moved to other sidebar without drag/drop isn’t trashed
Added more icon matchers. Cleaned up CSS a bit.

Cool.
Fixed a few issues with tab focusing
Merge pull request #75 from MichaelArestad/accessible-widget-reordering

Add more icon matchers, clean up CSS, improve focus states
Fix live previews for moving widget to another sidebar

* 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
Update availability of target sidebars when their render state changes
Focus on newly moved widget control

Focus on first focusable instead of on first visible input
Add theme support for twentyfourteen
Merge pull request #21 from x-team/accessible-widget-reordering

Add keyboard-accessible means of reordering widgets