widget/button,widget/material: introduce Clickable for generic click areas

material.Clickable is useful for adding a click response to any widget
or area.

Rename widget.Button to widget.Clickable to reflect the wider use
spectrum.

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

widget/button.go

@@ -13,8 +13,8 @@ import (
 	"gioui.org/op"
 )
 
-// Button represents a clickable area.
-type Button struct {
+// Clickable represents a clickable area.
+type Clickable struct {
 	click gesture.Click
 	// clicks tracks the number of unreported clicks.
 	clicks  int
@@ -30,7 +30,7 @@ type Click struct {
 // Clicked calls Update and reports whether the button was
 // clicked since the last call. Multiple clicks result in Clicked
 // returning true once per click.
-func (b *Button) Clicked(gtx *layout.Context) bool {
+func (b *Clickable) Clicked(gtx *layout.Context) bool {
 	b.Update(gtx)
 	if b.clicks > 0 {
 		b.clicks--
@@ -45,11 +45,11 @@ func (b *Button) Clicked(gtx *layout.Context) bool {
 
 // History is the past clicks useful for drawing click markers.
 // Clicks are retained for a short duration (about a second).
-func (b *Button) History() []Click {
+func (b *Clickable) History() []Click {
 	return b.history
 }
 
-func (b *Button) Layout(gtx *layout.Context) {
+func (b *Clickable) Layout(gtx *layout.Context) {
 	// Flush clicks from before the previous frame.
 	b.Update(gtx)
 	var st op.StackOp
@@ -69,7 +69,7 @@ func (b *Button) Layout(gtx *layout.Context) {
 
 // Update the button state by processing events. The underlying
 // gesture events are returned for use beyond what Clicked offers.
-func (b *Button) Update(gtx *layout.Context) []gesture.ClickEvent {
+func (b *Clickable) Update(gtx *layout.Context) []gesture.ClickEvent {
 	evts := b.click.Events(gtx)
 	for _, e := range evts {
 		switch e.Type {