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>

layout/layout.go

@@ -113,15 +113,15 @@ func (c Constraints) Constrain(size image.Point) image.Point {
 // constraints. The minimum constraints will be adjusted to ensure
 // they do not exceed the maximum.
 type Inset struct {
-	Top, Bottom, Left, Right unit.Value
+	Top, Bottom, Left, Right unit.Dp
 }
 
 // Layout a widget.
 func (in Inset) Layout(gtx Context, w Widget) Dimensions {
-	top := gtx.Px(in.Top)
-	right := gtx.Px(in.Right)
-	bottom := gtx.Px(in.Bottom)
-	left := gtx.Px(in.Left)
+	top := gtx.Dp(in.Top)
+	right := gtx.Dp(in.Right)
+	bottom := gtx.Dp(in.Bottom)
+	left := gtx.Dp(in.Left)
 	mcs := gtx.Constraints
 	mcs.Max.X -= left + right
 	if mcs.Max.X < 0 {
@@ -153,7 +153,7 @@ func (in Inset) Layout(gtx Context, w Widget) Dimensions {
 
 // UniformInset returns an Inset with a single inset applied to all
 // edges.
-func UniformInset(v unit.Value) Inset {
+func UniformInset(v unit.Dp) Inset {
 	return Inset{Top: v, Right: v, Bottom: v, Left: v}
 }
 
@@ -213,14 +213,14 @@ func (d Direction) Position(widget, bounds image.Point) image.Point {
 
 // Spacer adds space between widgets.
 type Spacer struct {
-	Width, Height unit.Value
+	Width, Height unit.Dp
 }
 
 func (s Spacer) Layout(gtx Context) Dimensions {
 	return Dimensions{
 		Size: image.Point{
-			X: gtx.Px(s.Width),
-			Y: gtx.Px(s.Height),
+			X: gtx.Dp(s.Width),
+			Y: gtx.Dp(s.Height),
 		},
 	}
 }