Add a setting for dialog headers

This setting will let us keep traditional appearance
of dialogs on platforms where this is expected.
The new setting is called gtk-dialogs-use-header, backed
by the Gtk/DialogsUseHeader xsetting.

gdk/x11/gdksettings.c

@@ -59,6 +59,7 @@ static const struct {
   {"Gtk/ShellShowsMenubar",   "gtk-shell-shows-menubar"},
   {"Gtk/ShellShowsDesktop",   "gtk-shell-shows-desktop"},
   {"Gtk/DecorationLayout",    "gtk-decoration-layout"},
+  {"Gtk/DialogsUseHeader",    "gtk-dialogs-use-header"},
   {"Gtk/EnablePrimaryPaste",  "gtk-enable-primary-paste"},
   {"Gtk/RecentFilesMaxAge",   "gtk-recent-files-max-age"},
   {"Gtk/RecentFilesEnabled",  "gtk-recent-files-enabled"},

gtk/gtksettings.c

@@ -215,6 +215,7 @@ enum {
   PROP_SHELL_SHOWS_MENUBAR,
   PROP_SHELL_SHOWS_DESKTOP,
   PROP_DECORATION_LAYOUT,
+  PROP_DIALOGS_USE_HEADER,
   PROP_ENABLE_PRIMARY_PASTE,
   PROP_RECENT_FILES_ENABLED
 };
@@ -1575,6 +1576,27 @@ gtk_settings_class_init (GtkSettingsClass *class)
                                              NULL);
   g_assert (result == PROP_DECORATION_LAYOUT);
 
+  /**
+   * GtkSettings:gtk-dialogs-use-header:
+   *
+   * Whether builtin GTK+ dialogs such as the file chooser, the
+   * color chooser or the font chooser will use a header bar at
+   * the top to show action widgets, or an action area at the bottom.
+   *
+   * This setting does not affect custom dialogs using GtkDialog
+   * directly, or message dialogs.
+   *
+   * Since: 3.12
+   */
+  result = settings_install_property_parser (class,
+                                             g_param_spec_boolean ("gtk-dialogs-use-header",
+                                                                   P_("Dialogs use header bar"),
+                                                                   P_("Whether builtin GTK+ dialogs should use a header bar instead of an action area."),
+                                                                   FALSE,
+                                                                   GTK_PARAM_READWRITE),
+                                             NULL);
+  g_assert (result == PROP_DIALOGS_USE_HEADER);
+
   /**
    * GtkSettings:gtk-enable-primary-paste:
    *