Nearest-Neighbor Filtering for resizing selections and images

[kinker31]

The title should be able to say it all, but ideally, I'd also like for it to work like it does in Paint.net, or even how Pinta handles anti-aliasing with it's paintbrush (and maybe another) tool.

[cameronwhite]

This was actually implemented recently for the next release, in 6e5545d, for resizing images