Moved GDNative singletons discoverer from register_types to
GDNativeSingletonEditor.
Fix enable/disable switch in GDNativeLibrarySingletonEditor.
Separate `gdnative/singletons` and `gdnative/singletons_disabled`
project settings, keeping "on by default" behavior.
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.
Fabio Alessandrelli
Rémi Verschelde
geequlim