all: [API] replace unit.Value with separate unit.Dp, unit.Sp types

The unit.Value is a struct and thus more inconvenient to use than its
underlying float32 type. In addition, most uses don't need a general
value, but rather a specific unit given by the context. This change
replaces unit.Value with two float32 units, Dp and Sp. It also changes
variables and parameters of unit.Value to a specific unit type matching
the context. That is, unit.Dp everywhere except for text sizes which are
in Sp.

Switching to typed float32s has multiple advantages

- They can be constants:

const touchSlop = unit.Dp(16)

- Casting untyped constants is no longer necessary:

insets := layout.UniformInset(16)

- Calculation with values is natural:

func (s ScrollbarStyle) Width() unit.Dp {
	return s.Indicator.MinorWidth + s.Track.MinorPadding + s.Track.MinorPadding
}

The main API change is that calls to gtx.Px must be replaced with either
gtx.Dp or gtx.Sp depending on the unit.

Idea by Christophe Meessen.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

widget/material/list.go

@@ -50,7 +50,7 @@ type ScrollTrackStyle struct {
 	// MajorPadding and MinorPadding along the major and minor axis of the
 	// scrollbar's track. This is used to keep the scrollbar from touching
 	// the edges of the content area.
-	MajorPadding, MinorPadding unit.Value
+	MajorPadding, MinorPadding unit.Dp
 	// Color of the track background.
 	Color color.NRGBA
 }
@@ -59,16 +59,16 @@ type ScrollTrackStyle struct {
 type ScrollIndicatorStyle struct {
 	// MajorMinLen is the smallest that the scroll indicator is allowed to
 	// be along the major axis.
-	MajorMinLen unit.Value
+	MajorMinLen unit.Dp
 	// MinorWidth is the width of the scroll indicator across the minor axis.
-	MinorWidth unit.Value
+	MinorWidth unit.Dp
 	// Color and HoverColor are the normal and hovered colors of the scroll
 	// indicator.
 	Color, HoverColor color.NRGBA
 	// CornerRadius is the corner radius of the rectangular indicator. 0
 	// will produce square corners. 0.5*MinorWidth will produce perfectly
 	// round corners.
-	CornerRadius unit.Value
+	CornerRadius unit.Dp
 }
 
 // ScrollbarStyle configures the presentation of a scrollbar.
@@ -89,13 +89,13 @@ func Scrollbar(th *Theme, state *widget.Scrollbar) ScrollbarStyle {
 	return ScrollbarStyle{
 		Scrollbar: state,
 		Track: ScrollTrackStyle{
-			MajorPadding: unit.Dp(2),
-			MinorPadding: unit.Dp(2),
+			MajorPadding: 2,
+			MinorPadding: 2,
 		},
 		Indicator: ScrollIndicatorStyle{
-			MajorMinLen:  unit.Dp(8),
-			MinorWidth:   unit.Dp(6),
-			CornerRadius: unit.Dp(3),
+			MajorMinLen:  8,
+			MinorWidth:   6,
+			CornerRadius: 3,
 			Color:        lightFg,
 			HoverColor:   darkFg,
 		},
@@ -104,8 +104,8 @@ func Scrollbar(th *Theme, state *widget.Scrollbar) ScrollbarStyle {
 
 // Width returns the minor axis width of the scrollbar in its current
 // configuration (taking padding for the scroll track into account).
-func (s ScrollbarStyle) Width(metric unit.Metric) unit.Value {
-	return unit.Add(metric, s.Indicator.MinorWidth, s.Track.MinorPadding, s.Track.MinorPadding)
+func (s ScrollbarStyle) Width() unit.Dp {
+	return s.Indicator.MinorWidth + s.Track.MinorPadding + s.Track.MinorPadding
 }
 
 // Layout the scrollbar.
@@ -119,7 +119,7 @@ func (s ScrollbarStyle) Layout(gtx layout.Context, axis layout.Axis, viewportSta
 	convert := axis.Convert
 	maxMajorAxis := convert(gtx.Constraints.Max).X
 	gtx.Constraints.Min.X = maxMajorAxis
-	gtx.Constraints.Min.Y = gtx.Px(s.Width(gtx.Metric))
+	gtx.Constraints.Min.Y = gtx.Dp(s.Width())
 	gtx.Constraints.Min = convert(gtx.Constraints.Min)
 	gtx.Constraints.Max = gtx.Constraints.Min
 
@@ -179,15 +179,15 @@ func (s ScrollbarStyle) layout(gtx layout.Context, axis layout.Axis, viewportSta
 				trackLen := gtx.Constraints.Min.X
 				viewStart := int(math.Round(float64(viewportStart) * float64(trackLen)))
 				viewEnd := int(math.Round(float64(viewportEnd) * float64(trackLen)))
-				indicatorLen := max(viewEnd-viewStart, gtx.Px(s.Indicator.MajorMinLen))
+				indicatorLen := max(viewEnd-viewStart, gtx.Dp(s.Indicator.MajorMinLen))
 				if viewStart+indicatorLen > trackLen {
 					viewStart = trackLen - indicatorLen
 				}
 				indicatorDims := axis.Convert(image.Point{
 					X: indicatorLen,
-					Y: gtx.Px(s.Indicator.MinorWidth),
+					Y: gtx.Dp(s.Indicator.MinorWidth),
 				})
-				radius := gtx.Px(s.Indicator.CornerRadius)
+				radius := gtx.Dp(s.Indicator.CornerRadius)
 
 				// Lay out the indicator.
 				offset := axis.Convert(image.Pt(viewStart, 0))
@@ -247,7 +247,7 @@ func (l ListStyle) Layout(gtx layout.Context, length int, w layout.ListElement)
 	originalConstraints := gtx.Constraints
 
 	// Determine how much space the scrollbar occupies.
-	barWidth := gtx.Px(l.Width(gtx.Metric))
+	barWidth := gtx.Dp(l.Width())
 
 	if l.AnchorStrategy == Occupy {