A couple of changes to Crossbow

I have made two changes to the original design by Tech2C in order to make it easier to fit the handle cover. The changes simply reduce the thickness of the Trigger and Firing Lever from 5mm to 4.2mm. I also wanted to reduce the thickness of the circular steps which sit at the bottom of the locating pins in the handle - my CAD program indicates that they are 0.5mm tall which works out to be two and a half layers when using a layer height of 0.2mm - impossible to print accurately with this settin