Support for referring to other globals from globals

- We need to make sure that the initialization of global is in the right order.
 - In C++ and rust, we need to add accessor to the global component
 - There can be `PropertyReference::Global` in binding of globals
 - The interpreter globals need to hold references to the global they may depend on

Fixes #175

api/cpp/cbindgen.rs

@@ -439,7 +439,8 @@ fn gen_backend_qt(
 
     config.export.body.insert(
         "NativeStyleMetrics".to_owned(),
-        "    inline NativeStyleMetrics(); inline ~NativeStyleMetrics();".to_owned(),
+        "    inline explicit NativeStyleMetrics(void* = nullptr); inline ~NativeStyleMetrics();"
+            .to_owned(),
     );
 
     let mut crate_dir = root_dir.to_owned();