On Windows 10, the creation of symlinks by normal users requires
enabling the developer mode, which may or may not be acceptable in
corporate environments with restricted IT setups.
We introduced the symlinks for the shared special key codes mapping,
which instead this patch places into a shared sixtyfps-common crate.
Share the code that defines the key with a macro over all the backends using
a symlink.
This is a symlink rather than exposing the macro directly since we add this
module in every backend, and each backend re-declares the macro to handle
the part that it needs. This needs to be a symlink because it will also be shared
with the compiler that does not depends on sixtyfps-corelib
