Add built-in ListBox copying with custom text and cancellation

Add built-in copying for desktop and Web ListBox cells so applications can copy ordinary cell text without writing a custom event handler. Provide an optional CellCopying event so applications that custom-draw cells can supply a meaningful text representation or cancel copying when no useful text representation exists.

For example, an ordinary name cell should copy its stored text automatically, while a custom-drawn status icon could copy "Online". A decorative image cell should be able to decline copying without changing the existing clipboard. Copying should respect text selection when the user is actively editing a cell.