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>

internal/fling/animation.go

@@ -7,7 +7,7 @@ import (
 	"runtime"
 	"time"
 
-	"gioui.org/ui"
+	"gioui.org/unit"
 )
 
 type Animation struct {
@@ -21,8 +21,8 @@ type Animation struct {
 
 var (
 	// Pixels/second.
-	minFlingVelocity = ui.Dp(50)
-	maxFlingVelocity = ui.Dp(8000)
+	minFlingVelocity = unit.Dp(50)
+	maxFlingVelocity = unit.Dp(8000)
 )
 
 const (
@@ -31,7 +31,7 @@ const (
 
 // Start a fling given a starting velocity. Returns whether a
 // fling was started.
-func (f *Animation) Start(c ui.Config, velocity float32) bool {
+func (f *Animation) Start(c unit.Converter, now time.Time, velocity float32) bool {
 	min := float32(c.Px(minFlingVelocity))
 	v := velocity
 	if -min <= v && v <= min {
@@ -43,7 +43,7 @@ func (f *Animation) Start(c ui.Config, velocity float32) bool {
 	} else if v < -max {
 		v = -max
 	}
-	f.init(c.Now(), v)
+	f.init(now, v)
 	return true
 }