exposes addButton and setValue

[jmej]

This functionality copies the work kabrio did in their fork:
master...kabrio:p5.gui:master

but updates setRangeValue to setValue so that it works with the latest quickSettings

[jmej]

This is pretty important functionality - it looks like @kabrio also did a pull request when the original work was done but it wasn't merged. I hope this gets merged to prevent future un-needed forks

[jmej]

this would close #21 #13 and #5

I wonder if it can be merged as a stop-gap while a more magic version is considered? I'm not sure what a magic version would exactly look like - so this feels like a practical solution.