After commit 3e5aa212d5 the Slint DLLs
aren't placed in the `bin/` directory by default anymore. Since the
tutorial builds Slint as an external sub-project, it is entirely
isolated and the CMAKE_*_OUTPUT_DIRECTORY variables do not propagate. On
macOS and Linux, the program still runs due to rpath. On Windows, the
instructions said to put `bin` into `%PATH%`, but that doesn't work
anymore, the dll is now in `_deps/slint-build`.
Instead of adjusting `%PATH%`, this change adjusts the documentation to
recommend the use of a custom command on Windows using
$<TARGET_RUNTIME_DLLS:tgt> to copy the DLL across. This is guarded with
WIN32 due to https://gitlab.kitware.com/cmake/cmake/-/issues/23543 ,
where the proposed solution requires CMake 3.26 (not released yet).
This is only required for dependencies of slint-build and xtask.
So this is not enforced yet and will still work with 1.56 with
for C++ or when not using slint-build.
Also update a few more links and reference to slint in the readme
I know this means that we will have invalid links and instructions until the release,
but the instructions are already broken anyway
The upcoming re-organization of the C++ header generation requires the
use of the CORROSION_ENVIRONMENT_VARIABLES target property, which in
turn requires CMake >= 3.19.
It does not update the version number in the README because
these are either not part of the versionized documentation
or the demantic versioning make it work anyway
