The item tree is intended to be initialized with pointers to the item
vtables, which are implemented using data relocation records that
resolve to the symbols exported by the sixtyfps_cpp shared library.
On windows, according to
https://docs.microsoft.com/en-us/cpp/c-language/rules-and-limitations-for-dllimport-dllexport?view=msvc-160
such data relocations are not supported, so this patch implements the
fallback through getter functions.