Fix rendering of gtk menus

* Fixed the linux `getMenuParent` to return the correct parent of the menu
* Fixed `AddMenu` and `AddMenuItem` to show the added menu/item
 
Menus now render natively when `Global.js` allows, however commands are not yet attached and the API is still not implemented properly. All this includes so far is fixes to make it work at the most basic level.

_Note: I'm learning GTK and C especially for this, so I might need a little bit of hand holding with the NativeMenuModel integration. I'll try to look into that soonish, but let me know if anyone else is working on it already :)_