Fixed: Error cause by attemptng to delete a NULL pointer.
unregister_gdnative_types() now checks discoverer to see if it is NULL
before deleting. After selecting a godot project to edit (in Win10), the
discoverer_callback() wasn't called thus discoverer was NULL.
A GDNativeLibrary now has a field "gdnative_singleton" which can be
used to let the `godot_gdnative_singleton` procedure be executed on
Godot's startup. In future this can be used to register new
scripting languages or resource importer types.
When looking up a symbol from a library, previously an error was
shown when the symbol did not exist. That caused confusion when the
lookup was completely optional.
This adds a new parameter to that method so that those errors can
be handled manually if needed.
This adds GDNative as a separate class type.
It can be used to interface with native libraries by
using "native calls", which can be registered by modules
(and in future other GDNative libraries?).
It also reworks the currently called "GDNativeScript" into a
"NativeScript" that just makes use of the new GDNative instead
of it being the component that implements that functionality.
