Remove test panel PTB dependency

[k1o0]

Is your feature request related to a problem? Please describe.
It's annoying to have to install PsychToolbox to run the Signals Test GUI. Instead, we could render the visual stimuli natively in a MATLAB UI Figure.

Describe the solution you'd like
The visual stimuli can be rendered in a MATLAB UI figure which is based on Web technology (it's a glorified browser window). I've tested it and WebGL is supported in MATLAB. Thus someone can simply port vis.init and vis.draw to this platform. This won't be suitable for real experiments though, only testing. We could then resurrect sig.test.playground for creating this figure.

Describe alternatives you've considered
Originally sig.playground worked by using the Java Open GL module but this was removed from MATLAB's dependencies some time ago and probably won't come back.

Additional context
If MATLAB was rewritten with uifigure , we could render stimuli live in mc. See #55 and #211.