[GDnative] Improve inheritance system in gdnative pluginscript

[touilleMan]

Currently parent class is passed to pluginscript by it name, which is not a good idea:

• this means there is no namespaces whatsoever (e.g. you cannot have vehicle/ai.xx and human/ai.xx, assuming .xx is a pluginscript-powered script language)
• it creates a mess if a script has the same name than a default Godot class (i.e. Node2D)

To solve this, pluginscript consider plain name as default Godot class, and otherwise wants a path (i.e. res://vehicle/ai.xx) as parent class attribute.
This way it even allow to inherit from another type of script !

ha ! and this PR also fix memory leaks with forgotten destructor. BTW I don't know if there is a better way to pass data returned from the binding without having to recreate another instance configured through pointer casting, I've tried to use reinterpret_cast but without any luck...

[touilleMan]

@karroffel sorry to ping you like that but given I updated the PR title to add the [GDNative] tag, you could have missed it...

Given we are very close to final 3.0, I would perfectly understand if this PR has to wait for 3.1 to be merged. However this would mean a breakage in the PluginScript API (parent script passed by name vs by path) which, depending of the success of PluginScript, can be something we want to avoid:

• if as of 3.1 release only Godot-Python uses PluginScript: we can break the API and no-one care ;-)
• if other module uses PluginScript: I can rework this PR to add a fallback (if the name is not found, try to search in the already defined pluginscript scripts before erroring)

[karroffel]

Looks okay to me :)

[touilleMan]

\o/

Let's hope this is my last PR to 3.0 now 😄