Option to delete wrongly added files when canceling the WYSIWYG embed stage

Adding a new file using the WYSIWYG plugin is a two-step process using the Upload or Web tab. First you add the file, then you embed it. However, it is only add the second stage you see a preview of the file and it is likely that the user then will discover that it was the wrong file and click cancel.

That will result in that the media browser closes and no file was embedded. The file though is still added to the library and unless the user knows this, they are unlikely to manually delete it using "admin/content/file/list".

We should in these cases give the user the option to delete or keep the file by asking "Do you also want to delete the file you just added" and two buttons "Delete" and "Keep".