Mike Fleetwood authored May 15, 2018 and Curtis Gedak committed Jun 18, 2018

The FileSystem Label dialog only has a single line of just 2 widgets; a
text label and entry box widget.  There is no need to use a multi-line
capable table to hold this.  Switch to a simpler horizontal box widget.

Note that this change is not related to porting to Gtk 3 and stopping
using deprecated APIs because both HBox [1] and Table [2] are deprecated
in Gtk 3.2 and Gtk 3.4 and replaced by Box with horizontal orientation
and Grid respectively.

[1] NEWS file from gtkmm 3.2, actually first released in gtkmm 3.1.6
    (unstable):
    https://git.gnome.org/browse/gtkmm/tree/NEWS?h=3.2.0#n91
        "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."

[2] NEWS file from gtkmm 3.4, actually first released in gtkmm 3.3.2
    (unstable):
    https://git.gnome.org/browse/gtkmm/tree/NEWS?h=3.4.0#n162
        "* Deprecate Gtk::Table in favour of Gtk::Grid."