Can't set item tooltip

[Mike Cookson <nick87720z gmail com>]

Trying to set tooltip for path, but can't get it working even for
rectangle. First i tried g_object_set() as most universal way. No
warnings and... dry :S

Also i tried goo_canvas_item_set_child_property (), but it seems, it is
rather for GooCanvasItem-based things, like same GooCanvasRect, which,
as warnd, don't have "tooltip" property.

P.S. imho, it is strange, that using of string in svg format is
alternative-less way to initialize items.