Introduced simple cheatsheet mode suitable for short lessons

[Harvie]

Hello, i am preparing short 45 minute lessons of 3d printing for kids at elementary school and i wanted to prepare limited cheatsheet for that ocassion. Therefore i've added javascript checkbox that hides everything i considered non-essential (by default everything is showing as before, unless you uncheck it):

Advanced: [x]

I wanted to keep it rather simple. Target is to teach the kids to model a "snowman". possibly with eyes, nose, hat and limbs based on their skill. that can be 3d printed. So i've only included the features that i use most of the time to create simple geometry.

[Harvie]

It's also useful to make elevator pitch about how friendly openscad learning curve is for beginners...