Improve filterability of GTP dissector.

Eliminated many, but not all proto_tree_add_text calls (mostly thanks to convert_proto_tree_add_text.pl)

Change-Id: I37d22ca9816ae1a3a42d911abeb73f7ba49f77d3
Reviewed-on: https://code.wireshark.org/review/4378
Reviewed-by: Michael Mann <mmann78@netscape.net>
Petri-Dish: Michael Mann <mmann78@netscape.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Anders Broman <a.broman58@gmail.com>