Apply selective clang-tidy suggestions

Currently translated at 100.0% (627 of 627 strings)

Co-authored-by: Linerly <linerly@protonmail.com>
Translate-URL: https://weblate.nheko.im/projects/nheko/nheko-master/id/
Translation: Nheko/nheko

Request inline images in the right size and anti-alias them

- If an inline image has specified a height, add parameters to the
  image:// URI.
- Add scaled to the parameters, the images would be cropped otherwise.
- Extract the height from image:// URI and use it for requestSize.
- Use scaledToHeight instead of scaled.

This adds a man page which lists command line options, key bindings and
commands. A new CMake option `MAN` is added, enabled by default on
`UNIX` but not `APPLE`.

The man pages are generated with either `a2x` (from asciidoc) or
`asciidoctor`. If both are not found, a fatal error is signaled.

Add note about secret server to Arch installation instructions

Might fix #844

See
https://code.qt.io/cgit/qt/qtdeclarative.git/commit/examples/quick/imageresponseprovider/imageresponseprovider.cpp?h=5.15&id=b1f238568214e6587b829d6695677e55a99b1d40
for context.

Currently translated at 100.0% (627 of 627 strings)

Co-authored-by: Linerly <linerly@protonmail.com>
Translate-URL: https://weblate.nheko.im/projects/nheko/nheko-master/id/
Translation: Nheko/nheko

relates to #845

Add openSUSE install instructions

Currently translated at 99.0% (621 of 627 strings)

Co-authored-by: Linerly <linerly@protonmail.com>
Translate-URL: https://weblate.nheko.im/projects/nheko/nheko-master/id/
Translation: Nheko/nheko

Currently translated at 100.0% (627 of 627 strings)

Co-authored-by: Priit Jõerüüt <nhkwlate@joeruut.com>
Translate-URL: https://weblate.nheko.im/projects/nheko/nheko-master/et/
Translation: Nheko/nheko

It seems like handling the message in a blocking manner is a no-go. I
have no idea how to fix that, so just use a queued connection for now...
(ASAN does not cooperate and just hides the crash D:)

fixes #842

Currently translated at 100.0% (627 of 627 strings)

Co-authored-by: Eldred HABERT <stuff@eldred.fr>
Translate-URL: https://weblate.nheko.im/projects/nheko/nheko-master/fr/
Translation: Nheko/nheko