Escape some characters like \n when appending text

This patch escapes some characters like \n when showing text
contained in a packet by appending it to a protocol item.

Change-Id: Ice0040040ec7ab573dd9a412f8c0c197a566a031
Reviewed-on: https://code.wireshark.org/review/3095
Petri-Dish: Michael Tüxen <tuexen@wireshark.org>
Reviewed-by: Michael Tüxen <tuexen@wireshark.org>