new PTA model-type, four-turtles appear in view

[#41923323]

partial list of what works:

PTA: patches, turtles, and agents ...

Though the only feature implemented is a *very* rudimentary
turtle feature so far ...

- new model-type 'pta' integrated w/interactive controller
- the turtles have a direction and velocity and move
- view container integrated with layout system
- code and model editors
- model serialization
- tick history
- benchmarks
PTA: add horizontal and vertical wrapping
four-turtles: use play-reset-step, lighten background
JavaScript mixin experiments
simplify PTA description of time
wip: pta shapes