Fixes #106868

This allows creating HTML presentations with
`asciidoctor-revealjs presentation.adoc`.

NOTE: The generated HTML file implicitly depend on reveal.js (and I see
no good way to bundle it with Nix). Either provide reveal.js in a local
path next to the generated HTML file[1], or override the revealjsdir
attribute, for example by pointing to an URL (can also be a path):

  asciidoctor-revealjs -a revealjsdir=https://cdn.jsdelivr.net/npm/reveal.js@3.9.2 presentation.adoc

Implementation details:
1. Added "gem 'asciidoctor-revealjs'" to the Gemfile.
2. Ran "nix-shell -p bundler --run 'bundle lock --update'"
   from pkgs/tools/typesetting/asciidoctor/.
3. Hand edited Gemfile.lock to remove all but the asciidoctor-revealjs
   changes. (Rationale: allow backporting to release-20.09.)
4. Finish off with "nix-shell -p bundix --run 'bundix'".

[1] Of course Nix can help with _that_, but that's external to the
asciidoctor-revealjs program.

(cherry picked from commit 8f13d3c)

This is a maintenance release of the Qt 5.15 release series. It focuses on
bug and security fixes. The feature that made me update this package was
the broken u2f support in QtWebEngine when used together with
Qutebrowser. It previously (on older Qt versions) used to work but
stopped working with the switch to 5.15.

As the changelog is rather extensive the reader is advised to inspect
[0] to checkout the changes to each of the components of their
interest.

This also fixes a security issue with QtWebEngine based browsers that
could be crashed by visiting a website that invokes certain WebRTC
features [1].

[0] https://wiki.qt.io/Qt_5.15.2_Change_Files
[1] https://bugreports.qt.io/browse/QTBUG-86752

(cherry picked from commit dd6446e)

Qt 5.15 carried a change that renamed a getter for the socket error. In
a later relase that change was reverted (to be less ambiguous) thus now
 those that had picked up the change in API had to change their code.
 This has now been reverted [0] and thus the consumers also have to
 revert their changes…

[0] https://codereview.qt-project.org/c/qt/qtbase/+/289693

(cherry picked from commit 5af26a1)

This keeps the PyQt5 version in sync with the Qt release. While it is
apparently not strictly required it is probably not the worst idea to do
this.

(cherry picked from commit a15b4c6)

(cherry picked from commit 2a46e71)

(cherry picked from commit 3ade461)

(cherry picked from commit 672e363)

Obsolete methods were still in use when 20.08 was released.
This patch ports away from them.

(cherry picked from commit cd8b364)

This reverts commit 8652352.

[20.09] gdb: Fix crash when exiting TUI with gdb -tui

[20.09] Qt 5.15.2

(cherry-picked from 22cd16f, basically)
Firefox 84.0.1 will require >= 3.59.1

(cherry picked from commit 85c8f75)

https://hydra.nixos.org/eval/1638217