Add overhang threshold for scarf joint seam

[SoftFever]

I didn't originally plan to add this feature to 2.0 as I wanted to do some more tests.
However, it turns out I don't have time to do more testing, so I will release it for more people to test.

[discip]

@SoftFever
Is the value supposed to be expressed in % (percent) rather than in ° (degree)?

[vgdh]

@SoftFever Is the value supposed to be expressed in % (percent) rather than in ° (degree)?

I think percent is more representative, because overhang percent may be different with the same incline degree because different line width.

[SoftFever]

Yeah, unsupported area of the perimeter can describe the overhang precisely. If we use angle, the overhang situation will be very different between 0.1 layer height to 0.3 layer height even if the angle is same.

[psiberfunk]

@SoftFever , I think this is a good compromise/hack for overhangs, however I think we might be able to get more fundamental improvement out of #4386. Have you considered if that's an easy or hard thing to do ?