x-end: refine integration with MK3 idler tensioner

x-end: refine integration with MK3 idler tensioner

 * Import slotted motor mount from MK3 upstream.
 * Remove idler tensioner and move the idler bearing into the x-end
   body.
 * Shift position of motor tensioner model to origin and make all
   parts relative to base body instead of being absolutely/statically
   positioned where it would need to go on the MK3 model. Needs a bit
   more tweaking since these x-ends are not quite the same dimensions,
   and will need further adjustment to scale automatically as the x-end
   size changes with the linear rod and bearing dimensions.