Provide ability to override EditorExportPlugin::_export_end() in C++

[Daylily-Zeleen]

I working on a custom godot engine which needing a custom exporter plugin in cpp.
However, unlike EditorExportPlugin::_export_file() and EditorExportPlugin::_export_begin(), EditorExportPlugin::_export_end() is private and can't be overrided to implement my logic directly in cpp.

I don't know this is intended or not, but in GDScript, these 3 methods can be override directly.
So I do a little change to let EditorExportPlugin::_export_end() can be overrided in cpp.

[akien-mga]

I was hoping for reduz to review as he designed this, but this didn't happen.

The changes seem sensible to me and consistent with how _export_begin is implemented, so this should be fine.

[akien-mga]

Thanks!