GtkSettings: add shell-shows-app-menu property

This is a boolean property that will be set to TRUE if the current
desktop environment is capable of displaying the application menu as
part of the desktop shell.

If it is FALSE then the application will need to display the menu for
itself.

gdk/x11/gdksettings.c

@@ -81,7 +81,8 @@ static const char gdk_settings_names[] =
   "Net/EnableEventSounds\0"   "gtk-enable-event-sounds\0"
   "Gtk/CursorBlinkTimeout\0"  "gtk-cursor-blink-timeout\0"
   "Gtk/AutoMnemonics\0"       "gtk-auto-mnemonics\0"
-  "Gtk/VisibleFocus\0"        "gtk-visible-focus\0";
+  "Gtk/VisibleFocus\0"        "gtk-visible-focus\0"
+  "Gtk/ShellShowsAppMenu\0"   "gtk-shell-shows-app-menu\0";
 
 
 static const struct
@@ -135,5 +136,6 @@ static const struct
   { 1636, 1658 },
   { 1682, 1705 },
   { 1730, 1748 },
-  { 1767, 1784 }
+  { 1767, 1784 },
+  { 1802, 1824 }
 };