Using this code to add button into media frame.
http://wordpress.stackexchange.com/a/88104
But its added to the WP builtin “Insert media” because of “wp.media.view.MediaFrame.Post” its added in code.
ACF image popup or gallery doesnt set a toolbar, but WP core fallback/default toolbar is “select”. My understanding here is that I would either need to create a separate toolbar or use some of the existing ones (for example the “select” one it already uses) but we miss some handlers for new buttons here.
