Media: Automatically select uploading attachments in single-select workflows.

The code now automatically selects uploading attachments in all workflows. In a workflow that selects multiple attachments, all attachments added to the upload queue are selected. In a workflow that selects a single attachment (e.g. selecting a featured image), the last attachment added to the upload queue is selected.

see #22817.


git-svn-id: http://core.svn.wordpress.org/trunk@23259 1a063a9b-81f0-0310-95a4-ce76da25c4cd