[gnome-mahjongg] Add F10 shortcut dialog.

[Arnaud B. <arnaudb src gnome org>]

commit 8e628bf3ff6dd736767e1cdf2c1c1df6c5b76ea9
Author: Arnaud Bonatti <arnaud bonatti gmail com>
Date:   Fri Oct 9 16:51:48 2020 +0200

    Add F10 shortcut dialog.
    
    Closes #19.

 data/help-overlay.ui    |  8 ++++++++
 src/gnome-mahjongg.vala | 10 +++++++++-
 2 files changed, 17 insertions(+), 1 deletion(-)
---
diff --git a/data/help-overlay.ui b/data/help-overlay.ui
index 61e7010..afc63dc 100644
--- a/data/help-overlay.ui
+++ b/data/help-overlay.ui
@@ -58,6 +58,14 @@
             <!-- Translators: title of a section in the Keyboard Shortcuts window; contains "Show Keyboard 
Shortcuts", "Show Help", "Quit"... -->
             <property name="title" translatable="yes" context="shortcut window">Generic</property>
             <property name="visible">1</property>
+            <child>
+              <object class="GtkShortcutsShortcut">
+                <property name="visible">1</property>
+                <property name="accelerator">F10</property>
+                <!-- Translators: F10 shortcut description in the Keyboard Shortcuts dialog, section 
Generic; show the main ("hamburger") menu -->
+                <property name="title" translatable="yes" context="shortcut window">Toggle main 
menu</property>
+              </object>
+            </child>
             <child>
               <object class="GtkShortcutsShortcut">
                 <property name="visible">1</property>
diff --git a/src/gnome-mahjongg.vala b/src/gnome-mahjongg.vala
index 77127d8..28ed6c8 100644
--- a/src/gnome-mahjongg.vala
+++ b/src/gnome-mahjongg.vala
@@ -18,6 +18,7 @@ public class Mahjongg : Gtk.Application
 
     private Gtk.ApplicationWindow window;
     private Gtk.Label title;
+    private Gtk.MenuButton menu_button;
     private int window_width;
     private int window_height;
     private bool is_maximized;
@@ -47,6 +48,7 @@ public class Mahjongg : Gtk.Application
         { "preferences",   preferences_cb  },
         { "help",          help_cb         },
         { "about",         about_cb        },
+        { "hamburger",     hamburger_cb    },
         { "quit",          quit_cb         }
     };
 
@@ -71,6 +73,7 @@ public class Mahjongg : Gtk.Application
         set_accels_for_action ("app.help",      {                 "F1"      });
         set_accels_for_action ("app.quit",      {        "<Primary>q",
                                                          "<Primary>w"       });
+        set_accels_for_action ("app.hamburger", {                 "F10"     });
 
         settings = new Settings ("org.gnome.Mahjongg");
 
@@ -156,7 +159,7 @@ public class Mahjongg : Gtk.Application
         menu.append_section (/* no title */ null, section);
         menu.freeze ();
 
-        var menu_button = new Gtk.MenuButton ();
+        menu_button = new Gtk.MenuButton ();
         menu_button.valign = Gtk.Align.CENTER;
         menu_button.set_menu_model (menu);
         menu_button.set_image (new Gtk.Image.from_icon_name ("open-menu-symbolic", Gtk.IconSize.BUTTON));
@@ -560,6 +563,11 @@ public class Mahjongg : Gtk.Application
         game_view.game.shuffle_remaining ();
     }
 
+    private inline void hamburger_cb ()
+    {
+        menu_button.active = !menu_button.active;
+    }
+
     private void about_cb ()
     {
         string[] authors =