Zoom: "Fit Image", "Fit Width", and "Fit Height" now available

This was a long-running request from many users, and a prerequisite to
adding drawing tools.  In 6.2's development cycle we laid the groundwork
for this, so dropping it in now was quite simple, all things considered.

The biggest complications arise from things like "zoom in" and "zoom
out" being used while a "fit" option is active.  I think all weird
edge-cases like this are handled correctly, but additional testing is
always welcome.