Plugin details: Accessibility improvements

* Focus the close link to transfer focus into the plugin modal.
* Set title attribute on iframe for screereaders.

props jwenerd for initial patch and a11y team for testing.
fixes #28823.
Built from https://develop.svn.wordpress.org/trunk@29534


git-svn-id: http://core.svn.wordpress.org/trunk@29310 1a063a9b-81f0-0310-95a4-ce76da25c4cd