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>

gesture/gesture.go

@@ -157,7 +157,7 @@ const (
 	StateFlinging
 )
 
-var touchSlop = unit.Dp(3)
+const touchSlop = unit.Dp(3)
 
 // Add the handler to the operation list to receive click events.
 func (c *Click) Add(ops *op.Ops) {
@@ -303,7 +303,7 @@ func (s *Scroll) Scroll(cfg unit.Metric, q event.Queue, t time.Time, axis Axis)
 				break
 			}
 			fling := s.estimator.Estimate()
-			if slop, d := float32(cfg.Px(touchSlop)), fling.Distance; d < -slop || d > slop {
+			if slop, d := float32(cfg.Dp(touchSlop)), fling.Distance; d < -slop || d > slop {
 				s.flinger.Start(cfg, t, fling.Velocity)
 			}
 			fallthrough
@@ -329,7 +329,7 @@ func (s *Scroll) Scroll(cfg unit.Metric, q event.Queue, t time.Time, axis Axis)
 			v := int(math.Round(float64(val)))
 			dist := s.last - v
 			if e.Priority < pointer.Grabbed {
-				slop := cfg.Px(touchSlop)
+				slop := cfg.Dp(touchSlop)
 				if dist := dist; dist >= slop || -slop >= dist {
 					s.grab = true
 				}
@@ -407,7 +407,7 @@ func (d *Drag) Events(cfg unit.Metric, q event.Queue, axis Axis) []pointer.Event
 			}
 			if e.Priority < pointer.Grabbed {
 				diff := e.Position.Sub(d.start)
-				slop := cfg.Px(touchSlop)
+				slop := cfg.Dp(touchSlop)
 				if diff.X*diff.X+diff.Y*diff.Y > float32(slop*slop) {
 					d.grab = true
 				}