[gnome-system-monitor] Added command-line help for gtk options (fixes #116)

[Robert Roth <robertroth src gnome org>]

commit 4036e0a2d54d91b57db6674de34e86e7f210b9ae
Author: Robert Roth <robert roth off gmail com>
Date:   Tue Oct 27 09:15:40 2020 +0200

    Added command-line help for gtk options (fixes #116)

 src/application.cpp | 4 ++++
 1 file changed, 4 insertions(+)
---
diff --git a/src/application.cpp b/src/application.cpp
index 1f3d7507..d8df911e 100644
--- a/src/application.cpp
+++ b/src/application.cpp
@@ -288,8 +288,12 @@ int GsmApplication::on_command_line(const Glib::RefPtr<Gio::ApplicationCommandLi
     Glib::OptionContext context;
     context.set_summary(_("A simple process and system monitor."));
     context.set_ignore_unknown_options(true);
+
+    Glib::OptionGroup gtkgroup(gtk_get_option_group(true));
     procman::OptionGroup option_group;
+
     context.set_main_group(option_group);
+    context.add_group (gtkgroup);
 
     try {
         context.parse(argc, argv);