widget: [API] move Clickable state update from Layout to Clicks

Before this change, Clickable state updates would happen in Layout.
However, that is too late in cases where clicks affects layout that
contiains the Clickable.

This change removes state changes from Layout and moves them to Clicks,
to allow users pre-layout access. Note that Layout itself processes
events, which means users can no longer access clicks after Layout.

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

widget/button_test.go

@@ -48,6 +48,7 @@ func TestClickable(t *testing.T) {
 		t.Error("button 2 should not have focus")
 	}
 	// frame: press & release return
+	frame()
 	r.Queue(
 		key.Event{
 			Name:  key.NameReturn,
@@ -58,11 +59,10 @@ func TestClickable(t *testing.T) {
 			State: key.Release,
 		},
 	)
-	frame()
-	if !b1.Clicked() {
+	if !b1.Clicked(gtx) {
 		t.Error("button 1 did not get clicked when it got return press & release")
 	}
-	if b2.Clicked() {
+	if b2.Clicked(gtx) {
 		t.Error("button 2 got clicked when it did not have focus")
 	}
 	// frame: press return down
@@ -73,7 +73,7 @@ func TestClickable(t *testing.T) {
 		},
 	)
 	frame()
-	if b1.Clicked() {
+	if b1.Clicked(gtx) {
 		t.Error("button 1 got clicked, even if it only got return press")
 	}
 	// frame: request focus for button 2
@@ -95,10 +95,10 @@ func TestClickable(t *testing.T) {
 		},
 	)
 	frame()
-	if b1.Clicked() {
+	if b1.Clicked(gtx) {
 		t.Error("button 1 got clicked, even if it had lost focus")
 	}
-	if b2.Clicked() {
+	if b2.Clicked(gtx) {
 		t.Error("button 2 should not have been clicked, as it only got return release")
 	}
 }