Improve the check all: click to check all checkboxes, click again to uncheck...

Improve the check all: click to check all checkboxes, click again to uncheck all, Shift/click to toggle. Shift/clicking two consecutive boxes checks the range between them as before. Fixes #8217

git-svn-id: https://core.svn.wordpress.org/trunk@9708 1a063a9b-81f0-0310-95a4-ce76da25c4cd