Add SetDotColor method

object.go

@@ -238,6 +238,8 @@ type TextBox interface {
 	// SetSelectable sets whether or not the text content can be
 	// highlighted/selected.
 	SetSelectable (bool)
+	// SetDotColor sets the highlight color of selected text.
+	SetDotColor   (color.Color)
 	// Select sets the text cursor or selection.
 	Select        (text.Dot)
 	// Dot returns the text cursor or selection.