PCB Vise for 1/4"-20 Camera thread mounts

This takes @ModelStation's excellent PCB Vise and @hesperaux's more printable jaw shapes, and adds an industry-standard 1/4"-20 threaded mount. My heavy camera ballhead is not going to move around on my desk or droop like those spindly little twist-and-ball helping hands always do.

I took @hesperaux's printable jaw and split it into its own STL so you can print single jaws for replacements, or arrange them differently on the print bed. The locking slide clip was also a bit loose even at the thick end, which may be down to printing tolerances, so I extended the slope just a smidge.