Motivation for this change

This change is a version bump for qutebrowser (and pdfjs). The following are the release notes:

Changed:

Windows/macOS releases now ship with Qt 5.12.6. This includes security fixes
up to Chromium 77.0.3865.120 plus a security fix for CVE-2019-13720 from
Chromium 78.

Fixed:

• Unbinding keys via config.bind(key, None) accidentally worked in v1.7.0 but raises an exception in v1.8.0. It now works again, but is deprecated and shows an error. Note that :config-py-write did write such invalid lines before v1.8.0, so existing config files might need adjustments.
• The readability-js userscript now handles encodings correctly (which it didn't before for some websites).
• can now be used to paste text starting with a hyphen.
• Following hints via the number keypad now works properly again.
• Errors while reading the state file are now displayed instead of causing a crash.
• Crash when using :debug-log-level without a console attached.
• Downloads are now hidden properly when the browser is in fullscreen mode.
• Crash when setting colors.webpage.bg to an empty value with QtWebKit.
• Crash when the history database file is not a proper sqlite database.
• Workaround for missing/broken error pages on Debian.
• A deprecation warning (caused by pywin32) about the imp module on Windows is now hidden.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

Notify maintainers

cc @