On macOS the clipboard actually returns an error when loading it and it
is empty. However this is not an `Empty` error but all errors are simple
boxed Errors from std.
Since loading the clipboard data usually should not fail, we now do not
log it as error if it fails but just print it to the debug log instead.
This fixes#2389.
This switches our own `copypasta` crate with the more standardized
`clipboard` library, which allows us to get rid of the `xclip`
dependency on X11.
Additionally, this lays the foundation for native Wayland clipboard
support once the clipboard crate is updated (or a fork is created).
Fixes#5.
Nathan Lilienthal