Widget Customizer: Move widget area sections to bottom.

Since a theme can have a lot of widget areas and we don't want to bury third-party sections, bump the widget sections to the bottom with a priority of 1000+. Fixes also the ordering.

props westonruter.
fixes #27401.

git-svn-id: https://develop.svn.wordpress.org/trunk@27541 602fd350-edb4-49c9-b593-d223f7449a82