moving egg_screen_* into libgnomeui

Hi Anders,
	Now that libgnome* has branched I'd like to move the
screen-exec wrappers from libegg into libgnome and libgnomeui.

	A reminder what they're for: all the added functions are
variants of the gnome-exec, gnome-help and gnome-url functions that
ensure the launched app appears on a given screen. The panel, applets
and nautilus all need these ...

	So here's what would be added to libgnomeui:

/* Utility methods for changing $DISPLAY to make
 * a given screen the default screen.
 */
char      *gnome_screen_exec_display_string        (GdkScreen     *screen);
char     **gnome_screen_exec_environment           (GdkScreen     *screen);


/* Multiscreen variants of the gnome-exec family
 */
int        gnome_screen_execute_async              (GdkScreen     *screen,
						    const char    *dir,
						    int            argc,
						    char * const   argv []);
int        gnome_screen_execute_shell              (GdkScreen     *screen,
						    const char    *dir,
						    const char    *command);
gboolean   gnome_screen_execute_command_line_async (GdkScreen     *screen,
						    const char    *command,
						    GError       **error);

/* Multiscreen variants of the gnome-help and gnome-url family
 */
gboolean   gnome_screen_url_show                   (GdkScreen     *screen,
						    const char    *url,
						    GError       **error);
gboolean   gnome_screen_help_display_uri           (GdkScreen      *screen,
						    const char    *help_uri,
						    GError       **error);
gboolean   gnome_screen_help_display               (GdkScreen     *screen,
						    const char    *file_name,
						    const char    *link_id,
						    GError       **error);
gboolean   gnome_screen_help_display_with_doc_id   (GdkScreen     *screen,
						    GnomeProgram  *program,
						    const char    *doc_id,
						    const char    *file_name,
						    const char    *link_id,
						    GError       **error);
gboolean   gnome_screen_help_display_desktop       (GdkScreen     *screen,
						    GnomeProgram  *program,
						    const char    *doc_id,
						    const char    *file_name,
						    const char    *link_id,
						    GError       **error);

	And to libgnome we would need to add variants of the gnome-url
and gnome-help methods which you can specify the enviroment of the
spawned viewer:

gboolean   gnome_url_show_with_env                 (const char    *url,
						    char         **envp,
						    GError       **error);
						    GError       **error);
gboolean   gnome_help_display_uri_with_env         (const char    *help_uri,
						    char         **envp,
gboolean   gnome_help_display_with_doc_id_with_env (GnomeProgram  *program,
						    const char    *doc_id,
						    const char    *file_name,
						    const char    *link_id,
						    char         **envp,
						    GError       **error);
gboolean   gnome_help_display_desktop_with_env     (GnomeProgram  *program,
						    const char    *doc_id,
						    const char    *file_name,
						    const char    *link_id,
						    char         **envp,
						    GError       **error);


	So, would you be okay with me going ahead with this?

Cheers,
Mark.