pdCanvas: add placeholder text + icon when no images are loaded

It would be nice to have clickable links that automatically raise an
Open dialog (or in the near future, a Browse dialog), but because that
requires the use of controls, it would need to be placed somewhere
outside the pdCanvas control.  Because that would require a lot of
program-wide logic to activate the separate control as necessary, I'm
postponing it until a later date.  This will do for now.