Allow expert info to show only items in the display filter.

This was started by Chris Maynard before Qt was announced and I just polished it off for inclusion in GTK version of Wireshark.

bug:1860
Change-Id: Icaada6e7900f22b0a3d97c2a5656edfd8d8c8b7f
Reviewed-on: https://code.wireshark.org/review/3035
Reviewed-by: Christopher Maynard <Christopher.Maynard@gtech.com>
Reviewed-by: Michael Mann <mmann78@netscape.net>
(cherry picked from commit d8cff57ef342b909f128244f3ab5e5debcec4988)
Reviewed-on: https://code.wireshark.org/review/3077