Display captured snapshots as thumbnails.

Display captured snapshots as thumbnails.

Once snapshots are captured after being generated by glretrace, they are
then displayed in the GUI app, qapitrace.

The retracer passes a list of thumbnails to the main window. The main
window in turn binds those thumbnails to the ApiTraceFrames for later
display.

When the ApiTrace events are displayed, if a frame is being display, its
thumbnail is queried. That thumbnail is then displayed at the
front of the line for a frame.