Canvas UC: fix progress bar z-order
Canvas UC: add failsafe when images can't be loaded from resource file

The IDE randomly throws an error when rendering the "drag an image" icon
to the back of the canvas; no idea why.  It's easy to catch and avoid,
at least...
Radio button and check box UC: auto-shrink font size of long captions as necessary

Phew, nice to have this done!  With this, the radio button and check box
controls *should* be ready for primetime.