example: bump gio version and add flat button example to kitchen

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

example/tabs/tabs.go

@@ -56,7 +56,7 @@ type Tabs struct {
 }
 
 type Tab struct {
-	btn   widget.Button
+	btn   widget.Clickable
 	Title string
 }