Tronxy X3 y axis and bed upgrade

I like the [design of Megreth](https://www.thingiverse.com/thing:2395455), but have problems with the t-nuts that I use and the holes diameters. So I reworked the holes and the tabs for better fit.
I also mixed the design of the bed with [hazevapecos solution](https://www.thingiverse.com/thing:2328473) of mounting the Igus bearings (two layers of kapton tape is wraped around the bearings for tighter fit).