Add an XSetting for the session bus ID

This will allow us to identify situations where we are not on
the same session bus as the xsettings provider.

https://bugzilla.gnome.org/show_bug.cgi?id=671802

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/SessionBusId",        "gtk-session-bus-id"},
   {"Gtk/DecorationLayout",    "gtk-decoration-layout"},
   {"Gtk/TitlebarDoubleClick", "gtk-titlebar-double-click"},
   {"Gtk/TitlebarMiddleClick", "gtk-titlebar-middle-click"},