Fix top widget padding and broken borders
Remove default live preview support from children of core themes
Close available widgets panel when previewed URL changes
Hide sidebar sections initially

Prevents FOIS (flash of inactive sidebars)

Fixes #36
Animate toggling of active/inactive sidebars

Fixes #9
Move logic for toggling customizer sections to parent

Eliminates use of cross-frame communication and instead uses customizer postMessage framework

Contributes toward #8
Auto-open sidebar section in customizer when accessed via notice on widgets admin

Fixes #36