Better support for custom DisplayObject subclasses

[MuTsunTsai]

This issue was previously mentioned in #90 which is now closed, so I suppose I open a new issue dedicate to it. The problem is if we have custom DisplayObject subclasses that is not part of pixi.js itself, the inspector will crash unless we register the class on the global PIXI object, for example:

class MyDisplayObject extends DisplayObject {
    ...
}

PIXI.MyDisplayObject = MyDisplayObject; // key!

This is necessary even for some semi-official packages, such as @pixi/graphics-smooth, as mentioned in its issue pixijs-userland/graphics-smooth#33.

It would be better if pixi-inspector could support custom classes directly without the need of such workarounds.

[MuTsunTsai]

Oh SORRY! I failed to update the extension to the latest version! My bad.