Fix inline image dialog javascript error

http://www.tinymce.com/develop/bugtracker_view.php?id=6717
Focus the editor after deleting the image so that the selection resets
and the old selection (where the image has the selection) isn't used,
which previously caused a javascript error on save.
Merge pull request #334 from monkeydiane/fixImageDialogError

Fix inline image dialog javascript error