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/checkbox.go

@@ -5,7 +5,6 @@ package material
 import (
 	"gioui.org/io/semantic"
 	"gioui.org/layout"
-	"gioui.org/unit"
 	"gioui.org/widget"
 )
 
@@ -21,8 +20,8 @@ func CheckBox(th *Theme, checkBox *widget.Bool, label string) CheckBoxStyle {
 			Label:              label,
 			Color:              th.Palette.Fg,
 			IconColor:          th.Palette.ContrastBg,
-			TextSize:           th.TextSize.Scale(14.0 / 16.0),
-			Size:               unit.Dp(26),
+			TextSize:           th.TextSize * 14.0 / 16.0,
+			Size:               26,
 			shaper:             th.Shaper,
 			checkedStateIcon:   th.Icon.CheckBoxChecked,
 			uncheckedStateIcon: th.Icon.CheckBoxUnchecked,