Qt: Show / hide main widgets.

Remove existing code which uses the visibilityChanged signal. It exists
in QToolbar but not QWidget. Use the actions to drive visibility
instead.

Update MainWindow::layoutPanes to respect "recent" settings.

Move the show / hide actions to the top of the View menu to match GTK+.

Change-Id: I670682e2a094945dbd36c80f43cd14515bbca5a6
Reviewed-on: https://code.wireshark.org/review/4911
Petri-Dish: Gerald Combs <gerald@wireshark.org>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Gerald Combs <gerald@wireshark.org>