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/image.go

@@ -39,7 +39,7 @@ func (im Image) Layout(gtx layout.Context) layout.Dimensions {
 
 	size := im.Src.Size()
 	wf, hf := float32(size.X), float32(size.Y)
-	w, h := gtx.Px(unit.Dp(wf*scale)), gtx.Px(unit.Dp(hf*scale))
+	w, h := gtx.Dp(unit.Dp(wf*scale)), gtx.Dp(unit.Dp(hf*scale))
 
 	dims, trans := im.Fit.scale(gtx.Constraints, im.Position, layout.Dimensions{Size: image.Pt(w, h)})
 	defer clip.Rect{Max: dims.Size}.Push(gtx.Ops).Pop()