the Halted exception gets a better home
re-wired the image export dialog

- still need to teach vm.export() to understand the new opts
made outputmode() into an Export to… option

- the export dialog box now has a cmyk checkbox (if appropriate)
- the command line tool now takes a --cmyk arg
- these set the mode at the beginning of the run, so if you call
outputmode() manually you can still override
- still need to add multipage-pdf handling to ImageSequence.m
added gaussians to random()