In the add media modal, fallback back to caption if alt is not set. Falback to...

In the add media modal, fallback back to caption if alt is not set. Falback to title if neither caption nor alt are set.

Props koopersmith
see #18984


git-svn-id: https://core.svn.wordpress.org/trunk@22937 1a063a9b-81f0-0310-95a4-ce76da25c4cd