Draft: Issue #6861: Symmetry Drawing: Decimal input bugs the symmetry.

Symmetry drawing allows decimal points for positioning the axes. This has the side effect of of producing an imperfect mirroring when the position contains a decimal point of .50.

Since it makes little sense to mirror at decimal points, this change introduces unsigned integers instead of doubles to position the axes.

I left the tiling symmetry tool untouched because it doesn't display axes, not sure if it makes sense to change it there.