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_js.go

@@ -643,10 +643,11 @@ func (w *window) draw(sync bool) {
 }
 
 func (w *window) getConfig() (image.Point, system.Insets, unit.Metric) {
+	invscale := unit.Dp(1. / w.scale)
 	return image.Pt(w.config.Size.X, w.config.Size.Y),
 		system.Insets{
-			Bottom: unit.Px(w.inset.Y),
-			Right:  unit.Px(w.inset.X),
+			Bottom: unit.Dp(w.inset.Y) * invscale,
+			Right:  unit.Dp(w.inset.X) * invscale,
 		}, unit.Metric{
 			PxPerDp: w.scale,
 			PxPerSp: w.scale,