Qt: Proto tree fixes

Qt: Proto tree fixes

Make sure ProtoTree::toString stops at the right place.

Don't emit fieldSelected when we're processing a context menu event. It
doesn't appear to be needed and it triggers unwanted automatic scrolling
when prefs.gui_auto_scroll_on_expand is enabled.

Change-Id: I351a6974d07aea6fdd3eb0b5c1975a1b8d8eb7da
Reviewed-on: https://code.wireshark.org/review/25396
Reviewed-by: Gerald Combs <gerald@wireshark.org>
Petri-Dish: Gerald Combs <gerald@wireshark.org>
Tested-by: Petri Dish Buildbot
Reviewed-by: Anders Broman <a.broman58@gmail.com>