Small fixes I just pushed

Hi all, pointed out by some students of mine, Magritte-Json had a small glitch where a getter was changed but the setter not accordingly (#useJsonKey) While fixing this I found than Magritte-Json did not run on Squeak correctly due to its Stack implementation being unreasonably limited. I fixed that by adding a Magritte-Squeak-Json package. Along the way, this makes Squeak compatible with Magritte3AddOns version 3.1 which I pushed as Magritte3AddOns version 3.1.3 (also including a Fix when loading the group #('Json') and not having loaded Magritte with tests beforehand). So: 1. small getter fix 2. Magritte3AddOns on Squeak, yay 3. Magritte3AddOns config glitch fixed. Hope that was in your interest :) best -Tobias