Release v1.0.0

* Breaking changes:
- Raise the minimum supported version of Emacs to 26.3 #26
  Drop support for Emacs 24 and 25. This allows for some code
  cleanup. *This is a major breaking change*.

- Change the default value of pdf-view-use-scaling #133
  `pdf-view-use-scaling` is now true by default, leading to rendering
  of crisp images on high-resolution screens. This should not cause
  problems on low-resolution screen (other than taking up more cache
  space / increasing rendering time), but if it does, please `(setq
  pdf-view-use-scaling nil)` in your configuration.

Other than the breaking changes, this release contains a lot of
changes related to user experience improvements and installation /
compatibility improvements.

It also sets up the stage for adding new features and closing
long-standing PRs into `pdf-tools`. I'm hoping to follow this release
up with a `1.1.0` release before the year is out, which will contain
some of these pending PR changes.

Thanks, and enjoy!
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