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/theme.go

@@ -34,7 +34,7 @@ type Palette struct {
 type Theme struct {
 	Shaper text.Shaper
 	Palette
-	TextSize unit.Value
+	TextSize unit.Sp
 	Icon     struct {
 		CheckBoxChecked   *widget.Icon
 		CheckBoxUnchecked *widget.Icon
@@ -43,7 +43,7 @@ type Theme struct {
 	}
 
 	// FingerSize is the minimum touch target size.
-	FingerSize unit.Value
+	FingerSize unit.Dp
 }
 
 func NewTheme(fontCollection []text.FontFace) *Theme {
@@ -56,7 +56,7 @@ func NewTheme(fontCollection []text.FontFace) *Theme {
 		ContrastBg: rgb(0x3f51b5),
 		ContrastFg: rgb(0xffffff),
 	}
-	t.TextSize = unit.Sp(16)
+	t.TextSize = 16
 
 	t.Icon.CheckBoxChecked = mustIcon(widget.NewIcon(icons.ToggleCheckBox))
 	t.Icon.CheckBoxUnchecked = mustIcon(widget.NewIcon(icons.ToggleCheckBoxOutlineBlank))
@@ -64,7 +64,7 @@ func NewTheme(fontCollection []text.FontFace) *Theme {
 	t.Icon.RadioUnchecked = mustIcon(widget.NewIcon(icons.ToggleRadioButtonUnchecked))
 
 	// 38dp is on the lower end of possible finger size.
-	t.FingerSize = unit.Dp(38)
+	t.FingerSize = 38
 
 	return t
 }