fdo#77987, part 1: set min&max values of spinboxes

by attaching adjustments to them. This restores pre-ui behaviour,
as without proper initial minimum value, one can't enter values < 0
into the dialog

I just copied the numbers as they were in .src file, but they were
prolly completely made up (and they're adjusted later anyway
according to size/position of workspace)

Change-Id: Ic09fdd9e947f04d6f2151e9d7a8714f4f1d29552
fdo#77987, part 2: snap line coordinates can be negative

thus, use signed integers here

Change-Id: I8de1662680bd6e5dc9d42d633fa591dacc260a9b