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/slider.go

@@ -31,19 +31,19 @@ type SliderStyle struct {
 	Color    color.NRGBA
 	Float    *widget.Float
 
-	FingerSize unit.Value
+	FingerSize unit.Dp
 }
 
 func (s SliderStyle) Layout(gtx layout.Context) layout.Dimensions {
-	thumbRadius := gtx.Px(unit.Dp(6))
-	trackWidth := gtx.Px(unit.Dp(2))
+	thumbRadius := gtx.Dp(6)
+	trackWidth := gtx.Dp(2)
 
 	axis := s.Float.Axis
 	// Keep a minimum length so that the track is always visible.
 	minLength := thumbRadius + 3*thumbRadius + thumbRadius
 	// Try to expand to finger size, but only if the constraints
 	// allow for it.
-	touchSizePx := min(gtx.Px(s.FingerSize), axis.Convert(gtx.Constraints.Max).Y)
+	touchSizePx := min(gtx.Dp(s.FingerSize), axis.Convert(gtx.Constraints.Max).Y)
 	sizeMain := max(axis.Convert(gtx.Constraints.Min).X, minLength)
 	sizeCross := max(2*thumbRadius, touchSizePx)
 	size := axis.Convert(image.Pt(sizeMain, sizeCross))