Mostly refactoring to belt_clips to enable reuse

Thanks for making the belt_clips part!  Was just what I needed to get my Kossel Mini working.  I got bored on an overseas flight and wanted to try my hand at making an integrated belt tensioner into the ball joint adapter, but only got to cleaning up the belt_clips code before the battery died.   **This pull request just removes duplication and makes the cutouts easier to put into other stuff later.**

[Semi-relevant to this commit, hoping to start a conversation around this part] The first few of these I printed w/a 0.5 nozzle at 0.5 infill, pretty quickly, and though they worked, under heavy tension the GT2 belt would slip.  I printed a second set much slower with a 0.35 nozzle and 0.2 layers instead of 0.4, plus 0.85 infill, and now I can't get the belt to slip.  I think a clamp with a screw lock is probably the way to go in the future, and also it would be nice if the clamp didn't get in the way of the arms and reduce the effective diameter.  I'm even tempted to move to a design that avoids using the teeth, simply because they require care to get right.