Twenty Fifteen: minor updates to readme.txt file for clarity and removing non-ASCII characters. Closes #30675.
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Customizer: Import missing `wp.customize.Control.prototype._toggleActive`.

Check also if `completeCallback` is set before calling it.

props westonruter.
see #30701.
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git-svn-id: http://core.svn.wordpress.org/trunk@30861 1a063a9b-81f0-0310-95a4-ce76da25c4cd