Modifications to gui_tools.py module. Enhancements to CaptureClicks
object and addition of CapturePolygons object. Onscreen help text can be
toggled on and off. CaptureClicks is a parent class of CapturePolygons.
CaptureClicks was refactored to separate drawing routines from event
handling. CapturePolygons is an easy tool for creating polygon regions
on an image.