Improve GObject introspection annotations (Fixes #8)

garcon-gtk/garcon-gtk-menu.c

garcon/garcon-config.c

garcon/garcon-menu-directory.c

garcon/garcon-menu-directory.h

garcon/garcon-menu-item-cache.c

garcon/garcon-menu-item-pool.c

garcon/garcon-menu-item.c

garcon/garcon-menu-merger.c

garcon/garcon-menu-node.c

garcon/garcon-menu-separator.c

garcon/garcon-menu-tree-provider.c

garcon/garcon-menu.c