Eliminate proto_tree_add_text from packet-spice.c

Also do some cleanup in dissect_spice() and replace g_warning calls.

Change-Id: I4dc6f6496bb5ae54157aa6f7f27f21c396da623c
Reviewed-on: https://code.wireshark.org/review/6106
Petri-Dish: Michael Mann <mmann78@netscape.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>