Quick change X Carriage for Mendel with LM8UU holders

It's a X Carriage with single screw quick changing of extruders/tools.
It has
* Strong LM8UU Holders
* T2.5 belt clamps (most other belt clamps should work fine)
* Mounts on each side for endstop flags or mounting a fan
* Universal adapter plate for using other extruders/tool mounts, which don't support quick changing.

The design reference and the raw plate are under CC0.

Version 2: Improved spacings