Fix a build break on Go < 1.14

This was the error:

ui/ui.go:517:2: duplicate method Length
ui/ui.go:546:39: rowFocusTableWidget.Length is ambiguous
ui/ui.go:546:39: cannot use res (type *rowFocusTableWidget) as type list.IBoundedWalker in argument to list.NewBounded:
	*rowFocusTableWidget does not implement list.IBoundedWalker (missing Length method)
ui/ui.go:640:39: psmlTableRowWidget.Length is ambiguous
ui/ui.go:640:39: cannot use res (type *psmlTableRowWidget) as type list.IBoundedWalker in argument to list.NewBounded:
	*psmlTableRowWidget does not implement
	list.IBoundedWalker (missing Length method)

Go 1.14 allows this ambiguity by taking the set union of interface
methods.

ui/ui.go

@@ -518,9 +518,9 @@ type iRowFocusTableWidgetNeeds interface {
 	table.IFocus
 	table.IGoToMiddle
 	table.ISetFocus
-	table.ISetPos
 	list.IWalkerHome
 	list.IWalkerEnd
+	SetPos(pos list.IBoundedWalkerPosition, app gowid.IApp)
 	FocusXY() (table.Coords, error)
 	SetFocusXY(gowid.IApp, table.Coords)
 	SetModel(table.IModel, gowid.IApp)