Add customizable dim transition times.

The default 400ms transition time from bright to black during dimming/undimming was a bit jarring, so I made that customizable with a slider.

The Hue API allows for 100ms increments, but I gave the slider a 500ms stepping under the assumption that some people might want long (~10s) transitions. That can easily be changed in settings.xml if it turns out users want more fine-grained control within a smaller spectrum.