Properly release added resource loader and save references. Otherwise PluginScript API
may cause "ObjectDB Instances still exist!" warnings and segmentation faults on exit.
This allows more consistency in the manner we include core headers,
where previously there would be a mix of absolute, relative and
include path-dependent includes.
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.
