Memory leak after unloaded Model and after unloaded its component by Loader.

While investigating memory usage when using PLY Model with Qt Quick 3D, I found the problem in the title.

Qt6.8.0 looks like it releases memory when the Model is unloaded, so please fix this in Qt6.5 and Qt5.15 as well, if possible.

The environment is as follows

The operations investigated for memory usage are as follows

1. Launch the application.
2. Repeat `Load`/`Unload` of `main.qml(Window)` three times.
3. After `Load` of `main.qml(Window)`, repeat `Unload` three times after the following operations.
   1. Repeat `Load`/`Unload` of `DisplayArea3D(including View3D)` three times.
4. After `Load` of `main.qml(Window)`, repeat `Unload` three times after the following operations.
   1. After `Load` of `DisplayArea3D (including View3D)`, repeat `Unload` three times after the following operations.
      1. Repeat `Load`/`Unload` of `PLY Model` 3 times.