Complete work on Command Bar

Barring bugs, I believe the new Command Bar tool is effectively
complete.  Full support has been added for reading/writing custom preset
values, and I used the complex Curves dialog to test everything - so we
should be good to go!

This update "fixes" #67, #68, and #75, at least theoretically.  I still
have to add the command bar control to each tool dialog, which will take
some time - but there are no longer any technical reasons barring those
issues from being treated as "resolved fixed".
Add Command Bar to Curves dialog