Media Grid: when closing the modal, automatically focus the proper attachment by reading the model's ID, which is unique.

Props adamsilverstein (for the red), wonderboymusic (for the green).
Fixes #28857.

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git-svn-id: http://core.svn.wordpress.org/trunk@29081 1a063a9b-81f0-0310-95a4-ce76da25c4cd