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.