Reinforced Prusa X Carriage with Slim LM8UU Holder

This is a reinforced Version of Simon's x carriage based on my slim LM8UU holders.

I basically changed the following:
* Beefed up the LM8UU clamps to 3mm wall thickness for better strength
* Added extra support braces for stiffer frame, also between the belt clamps to allow more belt tightness without twisting the clamps or the whole carriage
* Applied two fan mounts for multi-purpose fan or equipment installation (tiltable fan duct, LED ring light, etc.). Dimensions of mounting blocks are