Move the pointer to the "column_info" structure in the "frame_data"

Move the pointer to the "column_info" structure in the "frame_data"
structure to the "packet_info" structure; only stuff that's permanently
stored with each frame should be in the "frame_data" structure, and the
"column_info" structure is not guaranteed to hold the column values for
that frame at all times - it was only in the "frame_data" structure so
that it could be passed to dissectors, and, as all dissectors are now
passed a pointer to a "packet_info" structure, it could just as well be
put in the "packet_info" structure.

That saves memory, by shrinking the "frame_data" structure (there's one
of those per frame), and also lets us clean up the code a bit.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@4371 f5534014-38df-0310-8fa8-9805f1628bb7