Added a small widget framework to the interactive bevel SampleApp

I made a small framework to add slider and radial controls more easily.
The Sample now has controls for light direction and color.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2259183003

Review-Url: https://codereview.chromium.org/2259183003
1 file changed