Use .screen-reader-text class instead of an alt attribute for overlay button labels on Themes screen.

props bramd.
fixes #26603 for trunk.

git-svn-id: https://develop.svn.wordpress.org/trunk@27119 602fd350-edb4-49c9-b593-d223f7449a82
Twenty Fourteen: delete the `featured_content_ids` transient on theme switch to make sure child themes can override the Featured Content quantity value. Also remove quantity parameter so child themes can change the amount of featured posts with `$max_posts`.

Props obenland, see #26660.

git-svn-id: https://develop.svn.wordpress.org/trunk@27120 602fd350-edb4-49c9-b593-d223f7449a82