- add new config option `enable_osc52_clipboard_reading = false`
- add a new trait function:
`Clipboard::get_contents(&self, selection: ClipboardSelection, writer: Box<dyn ClipboardReader>)`

The clipboard content is read asynchronously by providing a sender
channel `Box<dyn ClipboardReader>`

The sender `ClipboardReader` is a trait that requires the channel to be
the `Send` + `Sync` + `Debug` (for the `MuxNotification` enum) along
with the special hackery trait `ClipboardReaderBoxClone` that allows `Box<dyn
ClipboardReader>)` to be `Clone`

Co-authored-by: Len Trigg <lenbok@gmail.com>

Co-authored-by: Len Trigg <lenbok@gmail.com>