visualgst: Do not display metaclasses in hierarchy browse.

When you're on the class browser select Object and show the
class hierarchy as a result you'll see all the metaclasses.

2013-06-02  Gwenael Casaccio  <mrgwen@gmail.com>

	* StBrowser/GtkClassHierarchyWidget.st: Fix GtkClassHierarchyWidget>>#classOrMeta:
	and GtkClassHierarchyWidget>>#buildTreeView.

Signed-off-by: Holger Hans Peter Freyther <holger@freyther.de>