A simple Stringer function for the expander widget

Helps debugging because it makes the children render their names when
printed with '%v', instead of a hex address for the widget and then no
further descent into the hierarchy.

widgets/expander/expander.go

@@ -8,6 +8,8 @@
 package expander
 
 import (
+	"fmt"
+
 	"github.com/gcla/gowid"
 	"github.com/gcla/gowid/widgets/boxadapter"
 )
@@ -61,6 +63,10 @@ func (w *Widget) Selectable() bool {
 	return w.w.Selectable()
 }
 
+func (w *Widget) String() string {
+	return fmt.Sprintf("expander[%v]", w.w.IWidget)
+}
+
 //======================================================================
 // Local Variables:
 // mode: Go