debug: Add GTK_DEBUG=snapshot

debug: Add GTK_DEBUG=snapshot

This causes the snapshotting algorithm to dump all widget nodes into
their own container node. We then name that group accordingly (ie
"GtkSwitch<0xdeadbeef>") so you can easily see which node belongs where.

The feature is toggleable in the inspector's visual tab.

There's a few problems with it, becuse GtkSnapshot optimized container
nodes away if they are not needed, so we are losing some widgets...