Instead of every snapshot function having debug messages, have an explicit gtk_snapshot_push_debug() function that appends a debug node.
This time, they are implemented as an overlay, so they require a running inspector and can't be enabled via env variable anymore.
Overlays are drawings that get rendered on top of the inspected window. The only overlay in existence so far is the highlight overlay, which is used to highlight widgets and replaces the "draw" signal handler used previously.
