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>

layout/example_test.go

@@ -6,7 +6,6 @@ import (
 
 	"gioui.org/layout"
 	"gioui.org/op"
-	"gioui.org/unit"
 )
 
 func ExampleInset() {
@@ -19,7 +18,7 @@ func ExampleInset() {
 	}
 
 	// Inset all edges by 10.
-	inset := layout.UniformInset(unit.Dp(10))
+	inset := layout.UniformInset(10)
 	dims := inset.Layout(gtx, func(gtx layout.Context) layout.Dimensions {
 		// Lay out a 50x50 sized widget.
 		dims := layoutWidget(gtx, 50, 50)