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/list_test.go

@@ -38,7 +38,7 @@ func TestListAnchorStrategies(t *testing.T) {
 		}
 		return layout.Dimensions{Size: image.Point{
 			X: gtx.Constraints.Max.X,
-			Y: gtx.Px(unit.Dp(20)),
+			Y: gtx.Dp(20),
 		}}
 	}
 
@@ -47,7 +47,7 @@ func TestListAnchorStrategies(t *testing.T) {
 	elements := 100
 	th := material.NewTheme(gofont.Collection())
 	materialList := material.List(th, &list)
-	indicatorWidth := gtx.Px(materialList.Width(gtx.Metric))
+	indicatorWidth := gtx.Dp(materialList.Width())
 
 	materialList.AnchorStrategy = material.Occupy
 	occupyDims := materialList.Layout(gtx, elements, space)