Hide Property<T> and PropertyTracker<T> from the public C++ API

Move those two classes into the private_api namespace, which is excluded
from the API reference documentation.

For generate code the explicit qualification of Property<T> is changed,
for the cbindgen generated item types the private_api namespace is
pulled into the cbindgen_private namespace.

api/sixtyfps-cpp/tests/datastructures.cpp

@@ -53,7 +53,7 @@ TEST_CASE("Basic SharedVector API", "[vector]")
 
 TEST_CASE("Property Tracker")
 {
-    using namespace sixtyfps;
+    using namespace sixtyfps::private_api;
     PropertyTracker tracker1;
     PropertyTracker tracker2;
     Property<int> prop(42);