resolve issues reported by dolf.

(but not all)

1. Don't zoom out when selecting output. Often you want to recheck an area you're working on... and you have to zoom back in again and again.

3. The distorted check is a great tool.. I have the default value at 45 degrees... I think you had it at 15? That's a bit low.

4. The upper limit for thickness check needs to be much higher. If you're working in mm 1 is too low (coloured prints have a lower limit of 2mm)... so make the limit for the check 10 or so.

5. Sharp edge check seems to work for both negative and positive values. There is very much a difference between those when printing... You're usually looking for pointy stuff.