Dropping files into the editor shouldn't insert file:///...

On some desktop environments, dragging and dropping a file gives us
the url to the file in multiple mime types, including text/plain. This
causes the url to be inserted as text by default.

Work around this issue by explicitly ignoring drop events for the text
editor when they also come with urls.

Task-number: QTCREATORBUG-728
Reviewed-by: Friedemann Kleint