Ensure trailing slash for customize-preview-js src

On a multisite subdirectory install, for a site 'foo', a URL
like the following results in a NOBLOGREDIRECT:

http://example.com/foo?styles-action=customize-preview-js...

This results in the preview not working at all. The fix is to ensure
that the script path ends in a slash, and this helps WordPress Multisite
properly route the request to the current site.
Store default setting value from JSON element

Currently the default value remains blank, so it does not show up in the controls
Improve handling of defaults for compound settings
Add background-position
Add support for background-position keywords (top, left, etc.)
Signed-off-by: Weston Ruter <weston@x-team.com>