Qt: Fix the filter expression toolbar layout.

Qt: Fix the filter expression toolbar layout.

Take a hint from the QToolButton sources and update the filter
expression toolbar using updateGeometry instead of adjustSize. This
appears to do the layout updates and other changes required to draw the
dropdown in the correct place.

Bug: 14121
Change-Id: I45d5a1173703f9c16d89841a57f651935da43d92
Reviewed-on: https://code.wireshark.org/review/25784
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>