Slider can now be controlled by scrolling

If ya nasty

slider.go

@@ -13,6 +13,7 @@ type Slider struct {
 	layout     sliderLayout
 	dragging   bool
 	dragOffset image.Point
+	step       float64
 
 	on struct {
 		slide event.FuncBroadcaster
@@ -34,6 +35,7 @@ func newSlider (orient string, value float64) *Slider {
 		layout: sliderLayout {
 			vertical: orient == "vertical",
 		},
+		step: 0.05,
 	}
 
 	this.Add(this.handle)
@@ -49,6 +51,7 @@ func newSlider (orient string, value float64) *Slider {
 	this.OnMouseDown(this.handleMouseDown)
 	this.OnMouseUp(this.handleMouseUp)
 	this.OnMouseMove(this.handleMouseMove)
+	this.OnScroll(this.handleScroll)
 	theme.Apply(this.handle, theme.R("objects", "SliderHandle", orient))
 	theme.Apply(this,        theme.R("objects", "Slider", orient))
 	return this
@@ -149,10 +152,10 @@ func (this *Slider) handleMouseDown (button input.Button) {
 		}
 	case input.ButtonRight:
 		if above {
-			this.SetValue(this.Value() - 0.05)
+			this.SetValue(this.Value() - this.step)
 			this.on.slide.Broadcast()
 		} else {
-			this.SetValue(this.Value() + 0.05)
+			this.SetValue(this.Value() + this.step)
 			this.on.slide.Broadcast()
 		}
 	}
@@ -168,6 +171,12 @@ func (this *Slider) handleMouseMove () {
 	this.drag()
 }
 
+func (this *Slider) handleScroll (x, y float64) {
+	delta := (x + y) * 0.005
+	this.SetValue(this.Value() + delta)
+	this.on.slide.Broadcast()
+}
+
 func (this *Slider) drag () {
 	pointer := this.MousePosition().Sub(this.dragOffset)
 	gutter  := this.InnerBounds()