This fixes various outdated links pointing to the old jwilm/alacritty
repository.
Since `copypasta` now has its own github repository at
https://github.com/alacritty/copypasta, the sources have been removed
from Alacritty.
To make the release process a bit smoother and prevent a freeze of the
master process while review candidates are out, this will put the master
in a perpetual development state.
This should make it clear to everyone that the official source for
releases is always the tagged branch and make it possible to release new
versions completely independently.
Since versions are bumped after each release, this makes it so the
release branches do not have to get merged back into the master branch
to show the correct development version.
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.
