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/context.go

@@ -52,15 +52,15 @@ func NewContext(ops *op.Ops, e system.FrameEvent) Context {
 	size := e.Size
 
 	if e.Insets != (system.Insets{}) {
-		left := e.Metric.Px(e.Insets.Left)
-		top := e.Metric.Px(e.Insets.Top)
+		left := e.Metric.Dp(e.Insets.Left)
+		top := e.Metric.Dp(e.Insets.Top)
 		op.Offset(image.Point{
 			X: left,
 			Y: top,
 		}).Add(ops)
 
-		size.X -= left + e.Metric.Px(e.Insets.Right)
-		size.Y -= top + e.Metric.Px(e.Insets.Bottom)
+		size.X -= left + e.Metric.Dp(e.Insets.Right)
+		size.Y -= top + e.Metric.Dp(e.Insets.Bottom)
 	}
 
 	return Context{
@@ -72,9 +72,14 @@ func NewContext(ops *op.Ops, e system.FrameEvent) Context {
 	}
 }
 
-// Px maps the value to pixels.
-func (c Context) Px(v unit.Value) int {
-	return c.Metric.Px(v)
+// Dp converts v to pixels.
+func (c Context) Dp(v unit.Dp) int {
+	return c.Metric.Dp(v)
+}
+
+// Sp converts v to pixels.
+func (c Context) Sp(v unit.Sp) int {
+	return c.Metric.Sp(v)
 }
 
 // Events returns the events available for the key. If no