This change makes the start of sharing the docs for the `TimerMode` enum
between Rust and C++. The reference to Timer::start in there works as
both doxygen and rustdoc find the right reference, but this needs
careful editing in the future and double-checking!
Another "caveat" is that the docs for the TimerMode enum say that the
enum is defined in the file "sixtyfps_generated_public.h", which is
correct as-is but not as pretty as "sixtyfps.h". I tried various ways
with \file and \includedoc, but couldn't get it working differently.
To implement this, the cppdocs steps now also runs cbindgen and cbindgen
generates a new sixtyfps_generated_public.h file that contains types we
do want to have in the public sixtyfps namespace.
Don't generate the headers by default in the source directory, put them
into a sub-directory in OUT_DIR instead and convey that location via
links to the C++ test driver.
Make install would still trigger the generated_headers_target CMake
custom command - due to the use of add_dependencies - and that would end
up running cbindgen. That in turn breaks "make install" when it's run in
an environment that doesn't have cargo in PATH (for example when using
"sudo make install").
This patch folds the generation of the C++ header files into the build
of the sixtyfps-cpp crate - via build.rs. By default the headers are
placed in api/sixtyfps-cpp/generated_include but the CMake build
redirects that into the build directory.
Note: Due to the way that corrosion works, cargo is still run when
running "make install", but it's path is absolute and there should not
be any reliance on the PATH environment variable.
