Deal with property encoding functions

Move everything dealing with compound text to be X11 specific
Only gdk_text_property_to_utf8_list and gdk_utf8_to_string_target
are kept across backends, so add vfuncs for these.

Also, remove the non-multihead-safe variants of all these.

gdk/x11/gdkprivate-x11.h

@@ -213,6 +213,15 @@ void        _gdk_x11_display_convert_selection      (GdkDisplay     *display,
                                                      GdkAtom         target,
                                                      guint32         time);
 
+gint        _gdk_x11_display_text_property_to_utf8_list (GdkDisplay     *display,
+                                                         GdkAtom         encoding,
+                                                         gint            format,
+                                                         const guchar   *text,
+                                                         gint            length,
+                                                         gchar        ***list);
+gchar *     _gdk_x11_display_utf8_to_string_target      (GdkDisplay     *displayt,
+                                                         const gchar    *str);
+
 void _gdk_x11_device_check_extension_events   (GdkDevice  *device);
 
 GdkDeviceManager *_gdk_x11_device_manager_new (GdkDisplay *display);