Add expert info configuration framework.  Bug 2412 (https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=2412).

Expert info "fields" can now be registered/addressed by name.  Right now, the basic framework allows expert info fields to become "display filters".  However more could be done, like user preferences overriding default severity level, speeding up expert info dialog load time by not needing to redissect a file, etc.

Long term goal is to have all expert_info filterable and have the functionality of expert_add_info_format() include the "registered index".  expert_add_info_format_text() is the workaround until all current calls to expert_add_info_format() have been updated with either expert_add_info() or expert_add_info_format_text().  Then the remaining expert_add_info_format_text() will be renamed to expert_add_info_format().

svn path=/trunk/; revision=49559