Switchplate with RGB sliders

Updated the excellent FixturePlate.scad so I could generate a wall plate for my RGB sliders.

Measurements are optimized for [Bourns PTA3043-2010CIB503](http://www.bourns.com/ProductLine.aspx?name=sliding_panel_controls) 30mm sliders, update the .scad file for other sizes. Also used Arial Rounded MT for the RGB labels.

I printed a draft version (pic attached) to check the measurements, will now test with a slower (4 hour!) version. I'm also going to upload a new scad file with the text bi