z-axis: improve strength, cleanup

z-axis: improve strength, cleanup

 * Use three point mounting for motors again, otherwise they can't be
   installed or removed without accessing the underside of the
   printer. Doesn't seem to affect stiffness in the end.

 * Increase strength of top bearing holder (intended to be printed
   with three perimeters)

 * Increase perimeter count for rod holder

 * Shift x-axis back in toward frame. The only reason to push it out
   was for the Wilson TS frame braces, which I am no longer
   using. Will eventually shift back in along x as well (need new
   x-ends first; z adjuster would hit the extrusions
   currently). AFAICT it only needs to be shifted outward in x to
   compensate for the 10mm travel lost by the endstop holder, which
   can be moved to under the x motor end as is done on the i3 MK2
   instead.

 * Take zaxis delta into account when generating final parts