Since at least Protobuf 3.21 the version is incompatible causing various
include and linker errors. Maybe the incompatibility was introduced before that,
but I don't know. If that is the case the version number can just be set lower
than that in the future.
The approach is pretty much just copied from ER-Forces framework
(https://github.com/robotics-erlangen/framework) and adapted to fit the
structure of this repository.

On some systems the packaged ODE (e.g. the Manjaro ode package at time of
writing) causes grSim to crash almost instantly.
This *may* be caused by the package having been built without double precision
support, but I don't know what exactly breaks.
Additionally add this as a hint to INSTALL.md.

… build protobuf correctly

Also changed vcpkg dependency from qt5 to qt5-base, because that should be
enough and vcpkg seems to build from source (?) and no one has time to build Qt
completely.
Includes some hacks to get around the fact that the macos pipeline does not set
the path to the QtConfig.cmake correctly.

Apparently some weird issues are caused by using the INSTALL_DIR parameter
of ExternalProjects when using the Ninja generator.
Makes you wonder why that is even there, but cmake works in mysterious ways,
I guess.
Adding the install part as a substep and explicitly telling cmake that the
necessary libs and executables are produced by this step fixes this issue.

Windows pipeline is now doing what the samples from run-vcpkg and run-cmake
suggest it should do and vcpkg claims that it should handle cmake find_package
calls correctly if you use their vcpkg.cmake as the toolchain.
It's not working, because for some reason the find_package call for Qt still
fails with cmake claiming it can't find a Qt5config.cmake file.

…t set