C++: Fix ABI mismatch with Slice on MSVC AArch64

The second constructor of `Slice` causes the Slice to be passed
differently (in different register) as a return value.

So remove that constructor and use a helper function to
construct a Slice

api/cpp/include/slint_window.h

@@ -216,8 +216,7 @@ public:
     inline std::optional<SharedString> register_font_from_data(const uint8_t *data, std::size_t len)
     {
         SharedString maybe_err;
-        cbindgen_private::slint_register_font_from_data(
-                &inner, { const_cast<uint8_t *>(data), len }, &maybe_err);
+        cbindgen_private::slint_register_font_from_data(&inner, make_slice(data, len), &maybe_err);
         if (!maybe_err.empty()) {
             return maybe_err;
         } else {