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/border.go

@@ -15,16 +15,16 @@ import (
 // Border lays out a widget and draws a border inside it.
 type Border struct {
 	Color        color.NRGBA
-	CornerRadius unit.Value
-	Width        unit.Value
+	CornerRadius unit.Dp
+	Width        unit.Dp
 }
 
 func (b Border) Layout(gtx layout.Context, w layout.Widget) layout.Dimensions {
 	dims := w(gtx)
 	sz := dims.Size
 
-	rr := gtx.Px(b.CornerRadius)
-	width := gtx.Px(b.Width)
+	rr := gtx.Dp(b.CornerRadius)
+	width := gtx.Dp(b.Width)
 	sz.X -= width
 	sz.Y -= width