BibFormat: plot file identification improvements

• Identify (and hide) "Plots" on the basis of their doctype, not based on file extension or caption. (closes #1514)

• Some CERN-specific handling of the files for the display.

Signed-off-by: Jerome Caffaro <jerome.caffaro@cern.ch>
Reviewed-by: Esteban J. G. Gabancho <esteban.gabancho@gmail.com>