Prefer higher priority translators even if they don't target the top frame

E.g. pdf+html pages for HighWire 2.0 (http://www.plantcell.org/content/25/4/1213.full.pdf+html?with-ds=yes)

I might be overlooking something, but I don't think this is currently possible (oversight on my part with the previous patch of course). The HighWire 2.0 translator is designed to attach to the sidebar frame and will return "`false`" for all other frames. Unfortunately, there is Embedded Metadata in the top frame, so EM takes over with the current selection criteria.