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