add a noCopy struct so go vet complains about copying widgets

widget/widget.go

@@ -9,8 +9,16 @@ import (
 	"fyne.io/fyne/v2/theme"
 )
 
+type noCopy struct{}
+
+func (*noCopy) Lock() {}
+
+func (*noCopy) Unlock() {}
+
 // BaseWidget provides a helper that handles basic widget behaviours.
 type BaseWidget struct {
+	noCopy // so `go vet` can complain if a widget is passed by value (copied)
+
 	size     fyne.Size
 	position fyne.Position
 	Hidden   bool