Add a GtkSetting for 'shell-shows-desktop'

Add a GtkSetting for whether the desktop shell is showing the desktop
folder icons.

This is on by default because most desktop shells do show the icons on
the desktop.  We already have a patch in gnome-settings-daemon to bind
this to the org.gnome.desktop.background show-desktop-icons GSettings
key which is off by default on GNOME.

https://bugzilla.gnome.org/show_bug.cgi?id=712302

gdk/wayland/gdkscreen-wayland.c

@@ -715,6 +715,9 @@ gdk_wayland_screen_get_setting (GdkScreen   *screen,
   if (strcmp (name, "gtk-shell-shows-menubar") == 0)
     return set_capability_setting (screen, value, GTK_SHELL_CAPABILITY_GLOBAL_MENU_BAR);
 
+  if (strcmp (name, "gtk-shell-shows-desktop") == 0)
+    return set_capability_setting (screen, value, GTK_SHELL_CAPABILITY_DESKTOP_ICONS);
+
   return FALSE;
 }
 

gdk/wayland/protocol/gtk-shell.xml

@@ -4,6 +4,7 @@
     <enum name="capability">
       <entry name="global_app_menu" value="1"/>
       <entry name="global_menu_bar" value="2"/>
+      <entry name="desktop_icons" value="3"/>
     </enum>
     
     <event name="capabilities">

gdk/x11/gdksettings.c

@@ -57,6 +57,7 @@ static const struct {
   {"Gtk/CursorBlinkTimeout",  "gtk-cursor-blink-timeout"},
   {"Gtk/ShellShowsAppMenu",   "gtk-shell-shows-app-menu"},
   {"Gtk/ShellShowsMenubar",   "gtk-shell-shows-menubar"},
+  {"Gtk/ShellShowsDesktop",   "gtk-shell-shows-desktop"},
   {"Gtk/EnablePrimaryPaste",  "gtk-enable-primary-paste"},
   {"Gtk/RecentFilesMaxAge",   "gtk-recent-files-max-age"},
   {"Gtk/RecentFilesEnabled",  "gtk-recent-files-enabled"},

gtk/gtksettings.c

@@ -209,6 +209,7 @@ enum {
   PROP_IM_STATUS_STYLE,
   PROP_SHELL_SHOWS_APP_MENU,
   PROP_SHELL_SHOWS_MENUBAR,
+  PROP_SHELL_SHOWS_DESKTOP,
   PROP_ENABLE_PRIMARY_PASTE,
   PROP_RECENT_FILES_ENABLED
 };
@@ -1527,6 +1528,16 @@ gtk_settings_class_init (GtkSettingsClass *class)
                                              NULL);
   g_assert (result == PROP_SHELL_SHOWS_MENUBAR);
 
+  result = settings_install_property_parser (class,
+                                             g_param_spec_boolean ("gtk-shell-shows-desktop",
+                                                                   P_("Desktop environment shows the desktop folder"),
+                                                                   P_("Set to TRUE if the desktop environment "
+                                                                      "is displaying the desktop folder, FALSE "
+                                                                      "if not."),
+                                                                   TRUE, GTK_PARAM_READWRITE),
+                                             NULL);
+  g_assert (result == PROP_SHELL_SHOWS_DESKTOP);
+
   /**
    * GtkSettings:gtk-enable-primary-paste:
    *