Creating a merge commit in order to create a 1.0.0 tag.

Note that this merge has breaking changes, as described in the NEWS
section.

* feature/emacs-26.3:
  autobuild: Recognize NetBSD and install packages via pkgin
  Bump the minimum Emacs version to 26.3! 🎉🤞
  Extend docker testing framework to test against Emacs versions
  Update and cleanup the Install section
  Make sure pkg-config is correctly set in autobuild
  Add support for Alpine Linux to autobuild
  Render crisp images for HiDPI screens by default
  Remove Emacs 24.4 guards for cua-mode
  Remove compatibility function for image-mode-winprops
  Remove pdf-util-window-pixel-width, fallback to window-body-width
  Remove macro / function re-definitions
  Remove bugfix for imenu in Emacs 24.3 and below
  Remove guards in `pdf-virtual` tests and code.
  Explicitly declare documentation files as Org files
  Add a byteclean target in the Makefile

Closes: #170

All the code working around issues in Emacs 24 and Emacs 25 has been
removed at this point! (or at least, as much of it as I could find)

I've tested against Emacs 28 and it's working fine. I need many more
automated tests to ensure that behaviour does not break across
multiple Emacs versions / operating systems, but that is a topic for
another day.

This commit bumps the version of `pdf-tools` from `1.0.0snapshot` to
`1.0.0`. I do not expect this to have any real change either on Melpa
Stable or Non-GNU Elpa, since they already parse the version as 1.0.0.

I am not creating a tag at the moment. I will do some more cleanup
work and directly create a `1.1.0` post those changes.

Closes: #26

Until this commit, docker-based local testing only checked whether the
epdfinfo server compiled correctly. With this commit, local testing
now also runs elisp tests against the different versions of Emacs we
currently support.

All versions of Emacs are run on Ubuntu at the moment,
but going forward I expect that we will add more operating system
variants to this matrix as well.

As test coverage increases, this will help automate away a lot of the
tests!

Relates to: #130

Simplify the instructions and ask users to use `make` where possible!

Closes: #160

Also, simplify the config script for finding gnu-sed.

Relates to: #160

This ensures that all operating systems currently mentioned in the
README are covered in the autobuild script

Relates to: #160

Copying from the README:

> `pdf-tools` version `1.1.0` release changed the default value of
> `pdf-view-use-scaling` to `t` (previously, it was `nil`). This has
> been done keeping in mind that most modern monitors are HiDPI
> screens, so the default configuration should cater to this user. If
> you are not using a HiDPI screen, you might have to change this
> value to `nil` in your configuration.

Closes: #133

Relates to: #26

Relates to: #26