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/icon.go

@@ -24,7 +24,7 @@ type Icon struct {
 	imgColor color.NRGBA
 }
 
-var defaultIconSize = unit.Dp(24)
+const defaultIconSize = unit.Dp(24)
 
 // NewIcon returns a new Icon from IconVG data.
 func NewIcon(data []byte) (*Icon, error) {
@@ -39,7 +39,7 @@ func NewIcon(data []byte) (*Icon, error) {
 func (ic *Icon) Layout(gtx layout.Context, color color.NRGBA) layout.Dimensions {
 	sz := gtx.Constraints.Min.X
 	if sz == 0 {
-		sz = gtx.Metric.Px(defaultIconSize)
+		sz = gtx.Dp(defaultIconSize)
 	}
 	size := gtx.Constraints.Constrain(image.Pt(sz, sz))
 	defer clip.Rect{Max: size}.Push(gtx.Ops).Pop()