Add a printer profile option to allow centered origin

[markwal]

This is an attempt at fixing #682. It doesn't add arbitrary origin, but adds a checkbox for the most common one (other than the default lower left). Some printers have (0, 0) at the center of the bed. Circular beds commonly, but also some rectangular beds.

[foosel]

Something's odd with the formatting here. I guess you wanted:

* - ``centeredOrigin``
  - ``bool``
  - Whether the printer's origin is in the center of the bed, otherwise the origin is assumed to be the lower left and all valid coordinates positive.

I'd rather move the option to volume.origin though, make it a string that accepts center, lowerleft and (later) x,y values (with center staying the default for a circular form factor).

[foosel]

That takes care of the settings and the gcode viewer, but it completely forgoes the slicer which needs that information as well.

Take a look here, here and here.

.oO( Maybe I should extract that "where to position the model" stuff somehow so it doesn't need to be reimplemented in every slicer )

[markwal]

Here's some updates based on feedback. I successfully sliced with the Cura addin for my printer which has the origin in the center.