The heuristic whether we're in the project manager inside GDMono
didn't work if the project manager was launched by not having any path
to run.
This is fixed now by making a Main::is_project_manager().
It's going to be called anyway after `mono_domain_finalize`.
This also prevents crashes, since the MessageQueue singleton could already be freed at this point (see: #15702).
Using `misc/scripts/fix_headers.py` on all Godot files.
Some missing header guards were added, and the header inclusion order
was fixed in the Bullet module.
- alloc_language_binding: Use strong GC handle as well for references. Fixes#15138
- Set the native instance field of Godot.Object to IntPtr.Zero when it's freed.
- Create weak handles without tracking resurrection (that was causing trouble). This means we have to call notification predelete before queueing a native Object for deletion, and use the MonoObject* passed by the finalizer because the weak GC handle target will return NULL at this point.
