Add: monocypher 4.0.2

Monocypher will take care of all our encryption needs; as most
OSes and vcpkg doesn't have it available, we vendor it.

Doc: Add more hyperlinks within the included documentation (#11288)

Co-authored-by: Niels Martin Hansen <nielsm@indvikleren.dk>

Feature: drop ICU-lx in favour of directly interfacing with harfbuzz

This means we have RTL support again with ICU 58+. It makes use of:
- ICU for bidi-itemization
- ICU for script-itemization
- OpenTTD for style-itemization
- harfbuzz for shaping

Add: Optional implementation of std::optional for pre-C++17 compilers.

Sourced from https://github.com/akrzemi1/Optional, Boost Software License, Version 1.0.

Remove: DOS support

In 10 years there was no active development on DOS. Although it
turned out to still work, the FPS was very bad. There is little
interest in the current community to look into this.

Further more, we like to switch to c++11 functions for threads,
which are not implemented by DJGPP, the only current compiler
for DOS.

Additionally, DOS is the only platform which does not support
networking. It is the reason we have tons of #ifdefs to support
disabling networking.

By removing DOS support, we can both use c++11 functions for threads,
and remove all the code related to disabling network. Sadly, this
means we have to see DOS go.

Of course, if you feel up for the task, simply revert this commit,
and implement stub c++11 functions for threads and stub functions
for networking. We are more than happy to accept such Pull Request.

Codechange: liblzma is called liblzma, how ever strange that might be

It is the only library we use that calls itself with 'lib' in the
name. This might be confusing, but with the arrival of cmake a lot
of these things are automated. And detection will find 'liblzma',
not 'lzma', like with 'lzo', 'zlib', ..

Remove: BeOS support (deprecated by Haiku)

In 10 years there is no commit to change how BeOS works, and we
have no active maintainer for it. It is unlikely it works in its
current state (but not impossible).

With the arrival of SDL2 (and removal of SDL), BeOS is no longer
support. SDL2 suggests to use Haiku instead of BeOS.

Remove: MorphOS / AmigaOS support

In 10 years there is no commit to change how MorphOS works, and we
have no active maintainer for it. It is unlikely it works in its
current state (but not impossible).

With the arrival of SDL2 (and removal of SDL), MorphOS is no longer
support. There is an SDL2 port for MorphOS, but it is not maintained
by upstream SDL2, and nobody can currently test it out.

If anyone wants to re-add MorphOS, please do (revert this patch,
fix the problems, and create a Pull Request). If you need any help
doing so, let us know! It is not that we don't like MorphOS, it is
that we don't have anyone fixing the problems :(

Update: Readme_Windows_MSVC documentation now reflects the current reality again

It was not updated for a while, and kinda out-of-date. This should
bring it back to reality.

Feature: Framerate display window (#6822)

Frame rate and various game loop/graphics timing measurements and graphs. Accessible via the Help menu, and can print some stats in the console via the fps command.

Update: Changed some things in Readme file

Changed HTTP links to HTTPS, fixed some links to articles on OpenTTD Wiki, added Windows 10 and 8.1 to info section about path structure in these OSes, removed some
unnecessary whitespaces (and added several others), and some other less important changes.