Update copyright email

Minor linting tweaks

Pending todo markers

Remove redundant return type

Only accept valid mime-types

Update dependencies

Mosty to address advisory in a transitive dependency:
https://rustsec.org/advisories/RUSTSEC-2021-0119

Update README

Update README

Add workaround for bug if Firefox's clipboard

Add scaffold for triggering notification.

Drop bogus TODO marker

Apply some clippy suggestions

Minor cleanups

Read from pipes without blocking

Drop TODO marker (doesn't make sense)

Drop os_pipe in favour of libc

The latter makes it very simple to create non-blocking pipes. We don't
care about non-unix environments anyway.

Support partially read selection data

We currently assume that non-empty data implies that we've finished
copying.

Drop this assumption in favour of an explicitly flag. This has little
impact on its own, but allows doing partial reads in future (which is
required in order to avoid doing blocking reads).

Drop verbose definition in favour of Default

Drop an unnecessary RefCell

This indirection isn't needed after all.

Replace `unwrap` with `expect`

Mostly to get clearer error messages. Also replace some usages of
`match` that could simply be `unwrap` or `expect`.