Saving datasets is now relative to scene path.

For ease of use, please try to always save your scene
file in a directory under which all your datasets are.

Now correctly using the name of the dataset saved in the
scene file when loading the scene.
Cleared the selection object list before loading.

Some quick cleanup.
Added management for case where source anatomy of VOI does not exist.

Also logging error when an invalid type is encountered.
Better error management on save.

The return value is now validated. In case of an error when saving any object,
the selection section of the scene file is now empty, and an error message box is shown.

Also added a “busy” icon when loading the scene.