feat: data device handling

[wash2]

This PR adds data device handling and an example demonstrating its use. I've also copied over the ReadPipe implementation from the previous version of sctk, because it seems useful. I haven't added very many abstractions, aside from the data sources, one for copy paste & another for drag and drop, to encourage that they are used correctly.

The example will read from the selection whenever it changes and can accept DnD. If the user presses left click, it will start a DnD. If the user presses "c" it will set the selection for copy paste.