Modular stackable alligator clip holder

This is a WIP for a modular alligator paint stick holder. These are stackable and can be joined together to secure them. Base model is 50mm x 100mm and sized for standard 3mm and 4mm skewers.

Edit 1: Modifications based on feed back
* Narrowed base slightly to allow better vertical stacking.
* Added small, medium, and large versions.
* Removed unneeded locks.
* Added a channel to make clearing support from the lock area easier.
* Added a quarter sized test version to print for checking fits.

Edit 2: Modifications for print time and fit
* Added a solid bottom for the first few mm to improve print time and adhesion on the larger models.
* Slightly widened the keyhole to apply less stress on the large model's double lock.