Redo header bar decorations once more

Applications need a way to fix or adapt the decoration layout,
for situations like split header bars. Setting the layout from
the theme with a style property did not offer a good way to do
this, and the ::show-close-button property does not provide
fine-grained control.

To improve the situation, move the layout string to a property of
GtkHeaderBar which is backed by a setting. This allows platforms to
set a default button layout independent of the theme, while applications
can override the default.

The style GtkWindow style property is now deprecated and ignored.

gdk/x11/gdksettings.c

@@ -58,6 +58,7 @@ static const struct {
   {"Gtk/ShellShowsAppMenu",   "gtk-shell-shows-app-menu"},
   {"Gtk/ShellShowsMenubar",   "gtk-shell-shows-menubar"},
   {"Gtk/ShellShowsDesktop",   "gtk-shell-shows-desktop"},
+  {"Gtk/DecorationLayout",    "gtk-decoration-layout"},
   {"Gtk/EnablePrimaryPaste",  "gtk-enable-primary-paste"},
   {"Gtk/RecentFilesMaxAge",   "gtk-recent-files-max-age"},
   {"Gtk/RecentFilesEnabled",  "gtk-recent-files-enabled"},