Drag/drop on the editor to upload: don't trigger the uploader when selected text is being dragged from one window to another.

Merges [28189] to the 3.9 branch.

props azaozz.
fixes #27880.

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git-svn-id: http://core.svn.wordpress.org/branches/3.9@28090 1a063a9b-81f0-0310-95a4-ce76da25c4cd