Turn the GTK+ 1.2.10 clist code into our own widget; that lets us modify it

Turn the GTK+ 1.2.10 clist code into our own widget; that lets us modify it
to add functionality or improve performance, although, until we make it
work on GTK+ 1.3[.x] and 2.x, we shouldn't count on the performance
improvements, or make its API different from that of the GtkClist API
(other than names).

Move all the code that knows about the packet list into
gtk/packet_list.c, so that the GtkClist vs. EthClist stuff is
encapsulated inside it.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@9608 f5534014-38df-0310-8fa8-9805f1628bb7