2000-05-21  Michael Natterer  <mitch@gimp.org>

	These changes enable help support for 3rd party plug-ins which
	install their help files outside GIMP's main help dir.

	Instead of calling gimp_help(), gimp_plugin_help_func() etc.,
	all help callbacks now have to call gimp_standard_help_func()
	which has different implementations in the app and in libgimp.

	There is a new function gimp_plugin_help_register() which can
	be called during plug-in query. plug_in.c keeps a list of
	executable_name/help_path pairs. Plug-ins have to pass their
	exec. name to gimp_help() which uses the list to find the plug-in's
	help directory.

	* app/gimphelp.[ch]: gimp_help() now takes a help_path parameter.
	help_path == NULL means the standard help directory. Various
	changes to pass the help_path to the help browser.

	* app/gimprc.c: save the plug-in's help_path in the pluginrc file.

	* app/menus.c: ugly hack to enable help_paths in the "F1" callback...