all: rename package ui to unit

Package ui is now only about units except for the Config.Now method.
Remove Now and rename Config to Converter. Add layout.Config to
replace the old ui.Config.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

layout/layout.go

@@ -4,10 +4,11 @@ package layout
 
 import (
 	"image"
+	"time"
 
 	"gioui.org/io/event"
 	"gioui.org/op"
-	"gioui.org/ui"
+	"gioui.org/unit"
 )
 
 // Constraints represent a set of acceptable ranges for
@@ -52,11 +53,20 @@ type Context struct {
 	// operation.
 	Dimensions Dimensions
 
-	ui.Config
+	Config
 	event.Queue
 	*op.Ops
 }
 
+// Config define the essential properties of
+// the environment.
+type Config interface {
+	// Now returns the current animation time.
+	Now() time.Time
+
+	unit.Converter
+}
+
 const (
 	Start Alignment = iota
 	End
@@ -93,7 +103,7 @@ func (s *Context) Layout(cs Constraints, w Widget) Dimensions {
 }
 
 // Reset the context.
-func (c *Context) Reset(cfg ui.Config, cs Constraints) {
+func (c *Context) Reset(cfg Config, cs Constraints) {
 	c.Constraints = cs
 	c.Dimensions = Dimensions{}
 	c.Config = cfg
@@ -129,7 +139,7 @@ func RigidConstraints(size image.Point) Constraints {
 
 // Inset adds space around a widget.
 type Inset struct {
-	Top, Right, Bottom, Left ui.Value
+	Top, Right, Bottom, Left unit.Value
 }
 
 // Align aligns a widget in the available space.
@@ -171,7 +181,7 @@ func (in Inset) Layout(gtx *Context, w Widget) {
 
 // UniformInset returns an Inset with a single inset applied to all
 // edges.
-func UniformInset(v ui.Value) Inset {
+func UniformInset(v unit.Value) Inset {
 	return Inset{Top: v, Right: v, Bottom: v, Left: v}
 }