Proportional editing: pixel radius option

The proportional editing radius is currently controlled with real-world units (and the default is very large relative to the default sizes for objects like cubes). It's often too large to be seen, which is confusing for beginners and inconvenient for experts.

It would be great to offer an option to set the proportional radius in pixels, rather than physical units. I also think the pixel radius should be the default, and the radius should be something reasonable that will be visible on most displays, but that's maybe slightly more controversial.