Qt: Display filter toolbar behavior tweaks.

Qt: Display filter toolbar behavior tweaks.

By default GtkToolbar shows overflow items in a menu. QToolBar shows
them in a bigger toolbar. This is a problem for users with lots of
display filter expressions because expanding the display filter toolbar
can make it very tall.

QToolBar can optionally overflow into a menu, but as noted in
https://bugreports.qt.io/browse/QTBUG-2472 there isn't a way do so
directly, e.g. with a setter. Force an overflow menu in the display
filter toolbar by adding a child QToolBar and adding filter expression
actions to it.

While we're here, set the minimum width of the display filter edit to
two-thirds of the width of the main window.

Change-Id: I49cd58397e087f1aa79b5bbce471f827e9804148
Reviewed-on: https://code.wireshark.org/review/15134
Petri-Dish: Gerald Combs <gerald@wireshark.org>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Gerald Combs <gerald@wireshark.org>