Import > Screenshot: sanitize window titles so that subsequent default Save As... filenames are valid
Noise > Bilateral Smoothing: optimizations and bug-fixes

Relates to #138.

Net gain from these optimizations ranges from 0% to ~50% time reduction
depending on the function parameters, so I'm afraid this isn't a "silver
bullet" for performance improvements.  (Large images + large radii can
still take 1+ minutes to process, alas.)

That said, I've been able to improve the function's accuracy in a number
of meaningful ways, while also fixing some potential bugs, so the net
result is an overall improved function.