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.
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git-svn-id: http://core.svn.wordpress.org/trunk@27384 1a063a9b-81f0-0310-95a4-ce76da25c4cd