SmartrapCore Alu X-Carriage with balanced belt tensioning

This is an x-carriage for the SmartRapCore Alu (and wooden version if using 8mm rods) printer. Based on the original part by Smartfriendz (http://smartfriendz.com/en/) and inspired by mbthor's (http://www.thingiverse.com/mbthor/about) design (http://www.thingiverse.com/thing:1566017) this design features a belt retaining mechanism which allows both belts to be simultaneously tensioned.

This alleviates the usual corexy problem of having to adjust the two belts to achieve square objects in the