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/label.go

@@ -96,9 +96,9 @@ func (p1 screenPos) Less(p2 screenPos) bool {
 	return p1.Y < p2.Y || (p1.Y == p2.Y && p1.X < p2.X)
 }
 
-func (l Label) Layout(gtx layout.Context, s text.Shaper, font text.Font, size unit.Value, txt string) layout.Dimensions {
+func (l Label) Layout(gtx layout.Context, s text.Shaper, font text.Font, size unit.Sp, txt string) layout.Dimensions {
 	cs := gtx.Constraints
-	textSize := fixed.I(gtx.Px(size))
+	textSize := fixed.I(gtx.Sp(size))
 	lines := s.LayoutString(font, textSize, cs.Max.X, gtx.Locale, txt)
 	if max := l.MaxLines; max > 0 && len(lines) > max {
 		lines = lines[:max]