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/editor.go

@@ -497,12 +497,12 @@ func (e *Editor) calculateViewSize(gtx layout.Context) image.Point {
 }
 
 // Layout lays out the editor. If content is not nil, it is laid out on top.
-func (e *Editor) Layout(gtx layout.Context, sh text.Shaper, font text.Font, size unit.Value, content layout.Widget) layout.Dimensions {
+func (e *Editor) Layout(gtx layout.Context, sh text.Shaper, font text.Font, size unit.Sp, content layout.Widget) layout.Dimensions {
 	if e.locale != gtx.Locale {
 		e.locale = gtx.Locale
 		e.invalidate()
 	}
-	textSize := fixed.I(gtx.Px(size))
+	textSize := fixed.I(gtx.Sp(size))
 	if e.font != font || e.textSize != textSize {
 		e.invalidate()
 		e.font = font
@@ -776,7 +776,7 @@ func (e *Editor) PaintText(gtx layout.Context) {
 // caretWidth returns the width occupied by the caret for the current
 // gtx.
 func (e *Editor) caretWidth(gtx layout.Context) int {
-	carWidth2 := gtx.Px(unit.Dp(1)) / 2
+	carWidth2 := gtx.Dp(1) / 2
 	if carWidth2 < 1 {
 		carWidth2 = 1
 	}