Add shell_write_string_to_stream()

Add a helper function to write a string as UTF-8 to a GOutputStream.
The signature of GOutputStream:

 gboolean g_output_stream_write_all (GOutputStream *stream,
                                     const void *buffer,
                                     gsize count,
                                     gsize *bytes_written,
                                     GCancellable *cancellable,
                                     GError **error);

Can't currently be handled by GJS.

https://bugzilla.gnome.org/show_bug.cgi?id=618189

src/shell-global.h

@@ -92,6 +92,10 @@ ClutterModifierType shell_get_event_state (ClutterEvent *event);
 void shell_popup_menu (GtkMenu *menu, int button, guint32 time,
                        int menu_x, int menu_y);
 
+gboolean shell_write_string_to_stream (GOutputStream *stream,
+                                       const char    *str,
+                                       GError       **error);
+
 guint32 shell_global_get_current_time (ShellGlobal *global);
 
 GAppLaunchContext *shell_global_create_app_launch_context (ShellGlobal *global);