Qt: TCP Stream Graphs dialog updates.

Qt: TCP Stream Graphs dialog updates.

Add duplicate ACK ticks to Statistics → TCP Stream Graphs → Time
Sequence (tcptrace), which I missed when porting from GTK+. Add zero
window crosses while we're here.

Switch TCPStreamDialog to a subclass of GeometryStateDialog.

Add a slot and URL for the Help button and a stub entry in the User's
Guide.

Bug: 12009
Change-Id: Idf2ddb9eb33d924d65998285b5cffc234156497c
Reviewed-on: https://code.wireshark.org/review/26592
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>