Add a 'quote_source_format' filter and make the default an em-dash and a space preceding the source.

fixes #24243

git-svn-id: http://core.svn.wordpress.org/trunk@24170 1a063a9b-81f0-0310-95a4-ce76da25c4cd
POT, generated from r24170

git-svn-id: http://core.svn.wordpress.org/trunk@24171 1a063a9b-81f0-0310-95a4-ce76da25c4cd