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>

internal/fling/animation.go

@@ -19,25 +19,22 @@ type Animation struct {
 	v0 float32
 }
 
-var (
-	// Pixels/second.
-	minFlingVelocity = unit.Dp(50)
-	maxFlingVelocity = unit.Dp(8000)
-)
-
 const (
+	// dp/second.
+	minFlingVelocity  = unit.Dp(50)
+	maxFlingVelocity  = unit.Dp(8000)
 	thresholdVelocity = 1
 )
 
 // Start a fling given a starting velocity. Returns whether a
 // fling was started.
 func (f *Animation) Start(c unit.Metric, now time.Time, velocity float32) bool {
-	min := float32(c.Px(minFlingVelocity))
+	min := float32(c.Dp(minFlingVelocity))
 	v := velocity
 	if -min <= v && v <= min {
 		return false
 	}
-	max := float32(c.Px(maxFlingVelocity))
+	max := float32(c.Dp(maxFlingVelocity))
 	if v > max {
 		v = max
 	} else if v < -max {