Luca Bacci authored Feb 27, 2019 and Mike Fleetwood committed Apr 27, 2019

Gtk::HBox and Gtk::VBox were deprecated in Gtkmm 3.2 [1].  Replace with
plain Gtk::Box.

This commit makes the change for Win_GParted.{h,cc}.

[1] Gtkmm 3.2.0 NEWS file (actually first included in gtkmm 3.1.6
    unstable)
    https://gitlab.gnome.org/GNOME/gtkmm/blob/3.2.0/NEWS#L91
        Gtk:
        * All H* or V* specialized classes have been deprecated, to
          match the deprecations in the GTK+ C API.  You should now
          set the orientation instead.
          This includes HBox, VBox, HButtonBox, VButtonBox, HPaned,
          VPaned, HScale, VScale, HSeparator, VSeparator, HScrollbar and
          VScrollbar.

Closes !25 - Modern Gtk3 - part 1