Generate fully qualified names for global c++ classes

**Summary:**
For hxcpp classes not defined in namespace, e.g. `::String`, `::Array` and `::Dynamic`, the compiler should always generate them with the `::` prefix so they are fully qualified.

**Rationale:**
Working with godot lately and they have a C++ class`godot::String`.
If we also declare a Haxe class like this:

```haxe
package godot;

class Foo {
public function print(str:String) {}
}
```

the generated c++ code will look somewhat like this:

```cpp
namespace godot {
class Foo {
void print(str:String);
}
}
```

and the c++ compiler would think that the function argument is a `godot::String` and causing c++ compilation errors.

**Alternatively:**
All these global hxcpp class should be moved to the hx namespace to be consistent with other classes.