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_test.go

@@ -7,7 +7,7 @@ import (
 
 	"gioui.org/io/event"
 	"gioui.org/layout"
-	"gioui.org/ui"
+	"gioui.org/unit"
 )
 
 type queue struct{}
@@ -26,7 +26,7 @@ func ExampleInset() {
 	gtx.Reset(cfg, cs)
 
 	// Inset all edges by 10.
-	inset := layout.UniformInset(ui.Dp(10))
+	inset := layout.UniformInset(unit.Dp(10))
 	inset.Layout(gtx, func() {
 		// Lay out a 50x50 sized widget.
 		layoutWidget(gtx, 50, 50)
@@ -146,7 +146,7 @@ func (config) Now() time.Time {
 	return time.Now()
 }
 
-func (config) Px(v ui.Value) int {
+func (config) Px(v unit.Value) int {
 	return int(v.V + .5)
 }