Generate fully qualified names for global c++ classes

[kevinresol]

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++ classgodot::String.
If we also declare a Haxe class like this:

package godot;

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

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

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.