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>

app/os_ios.go

@@ -150,6 +150,11 @@ func (w *window) draw(sync bool) {
 		w.w.Event(system.StageEvent{Stage: system.StageRunning})
 	}
 	const inchPrDp = 1.0 / 163
+	m := unit.Metric{
+		PxPerDp: float32(params.dpi) * inchPrDp,
+		PxPerSp: float32(params.sdpi) * inchPrDp,
+	}
+	dppp := unit.Dp(1. / m.PxPerDp)
 	w.w.Event(frameEvent{
 		FrameEvent: system.FrameEvent{
 			Now: time.Now(),
@@ -158,15 +163,12 @@ func (w *window) draw(sync bool) {
 				Y: int(params.height + .5),
 			},
 			Insets: system.Insets{
-				Top:    unit.Px(float32(params.top)),
-				Bottom: unit.Px(float32(params.bottom)),
-				Left:   unit.Px(float32(params.left)),
-				Right:  unit.Px(float32(params.right)),
-			},
-			Metric: unit.Metric{
-				PxPerDp: float32(params.dpi) * inchPrDp,
-				PxPerSp: float32(params.sdpi) * inchPrDp,
+				Top:    unit.Dp(params.top) * dppp,
+				Bottom: unit.Dp(params.bottom) * dppp,
+				Left:   unit.Dp(params.left) * dppp,
+				Right:  unit.Dp(params.right) * dppp,
 			},
+			Metric: m,
 		},
 		Sync: sync,
 	})