Hide/move QtQuick3D's Skeleton, Joint, and MorphTarget components in Component Library

It seems that there is very few real world use cases for manually adding Skeleton, Joint, and MorphTarget components into the scene. These all require compatible mesh, so normally they will be generated as part of the import process.

You may need to manually craft .qml files containing these if you use custom geometry that uses these, but creating custom geometries requires C++, so it is unlikely someone creating a module containing those creates corresponding .qml files using QDS.

Double check from qtquick3d team if there are some other use cases where you might need to have these available in the library.

If there is no practical use cases for manually adding these components to regular scenes, I propose we either hide them in the component library to reduce clutter, or at least move them under separate category in the library (e.g. "Qt Quick 3D Custom Geometry Utils" or something, similar to custom shader utils category we already have.

Even if the library items are hidden, the property sheets should still be available for users looking at imported .qml files.