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/loader.go

@@ -13,7 +13,6 @@ import (
 	"gioui.org/op"
 	"gioui.org/op/clip"
 	"gioui.org/op/paint"
-	"gioui.org/unit"
 )
 
 type LoaderStyle struct {
@@ -32,7 +31,7 @@ func (l LoaderStyle) Layout(gtx layout.Context) layout.Dimensions {
 		diam = minY
 	}
 	if diam == 0 {
-		diam = gtx.Px(unit.Dp(24))
+		diam = gtx.Dp(24)
 	}
 	sz := gtx.Constraints.Constrain(image.Pt(diam, diam))
 	radius := sz.X / 2