Allow nodes to graphically represent whether they have viewable descendents

[GoogleCodeExporter]

Feature request:

For container nodes, use two different icons. One icon should 
represent "some of my descendents have data that can be displayed in the 
viewer". The other icon should represent "none of my descendents have data 
that can be displayed in the viewer". The first icon could be the one 
currently used; the second could be a dimmed version of the first.

This will allow casual/new users who are not familiar with pigg 
organization to more easily locate things that they can actually look at.

Original issue reported on code.google.com by [email protected] on 11 Sep 2009 at 5:04