Button: Color always grey regardless of fill color parameter

[RebeccaHirst]

The Color of buttons is always grey regardless of the value in the Fill Color field of the component.

Minimal demo here: https://pavlovia.org/Consultancy/buttoncolorbug

[thewhodidthis]

Thanks @RebeccaHirst, this looks like a boilerplate issue. I have forked your project to demostrate the potential workaround of manually setting fillColor on the ButtonStim instance, but definitely we should look into tweaking PsychoPy's Builder to expand on the options given when calling the ButtonStim constructor.