Luca Bacci authored Aug 09, 2018 and Mike Fleetwood committed Feb 11, 2019

gdk_spawn_command_line_on_screen() is not present in Gtk3.  The
documentation from Gtkmm 2.24 states [1]:

    gdk_spawn_command_line_on_screen has been deprecated since version
    2.24 and should not be used in newly-written code.  This function is
    being removed in 3.0.  Use either g_spawn_command_line_sync(),
    g_spawn_command_line_async() or GdkAppLaunchContext instead.

g_spawn_command_line_sync() and g_spawn_command_line_async() are screen
/ display agnostic, as such we would loose functionality.  There is a
workaround, which involves setting the DISPLAY environment variable [2],
but it's a weak solution (and I don't know if it works on backends
other than X11).

GdkAppLaunchContext is an implementation of GIO's GAppLaunchContext that
handles launching an application in a graphical context [3].  Therefore
use GdkAppLaunchContext and GIO's GAppInfo.

GdkAppLaunchContext was introduced in Gtk2 version 2.14.  The C++
wrapper Gdk::AppLaunchContext was introduced only in Gtkmm3 version 3.4
[4].  Bump the minimum required version of Gtkmm to 3.4.0 for this
requirement.

GAppInfo was introduced in GLib version 2.16.  The C++ wrapper
Gio::AppInfo was introduced in Giomm version 2.16.  Note that the
minimum required version for glibmm is already 2.32.

[1] GDK 2 Reference Manual, GdkScreen, gdk_spawn_on_screen()
    https://developer.gnome.org/gdk2/2.24/GdkScreen.html#gdk-spawn-on-screen

[2] Migrating from GTK+ 2.x to GTK+ 3 - "Use GIO for launching applications"
    https://developer.gnome.org/gtk3/stable/gtk-migrating-2-to-3.html#id-1.6.3.3.7

[3] GDK 3 Reference Manual - "Application launching"
    https://developer.gnome.org/gdk3/stable/gdk3-Application-launching.html

[4] Gtkmm 3.4 Gdk::AppLaunchContext Class Reference, Detailed Description
    https://developer.gnome.org/gtkmm/3.4/classGdk_1_1AppLaunchContext.html#details

Closes #7 - Port to Gtk3