Volumes may not be fully deleted

[kephale]

Tested by:
running in intellij, ImageJMain (on ubuntu)
ImageJ bar: open samples > T1 Head
sciview: add volume
sciview: select volume in inspector
sciview: delete volume

The inspector still has attributes open for the volume
@skalarproduktraum

[kephale]

If the object that is currently selected gets deleted, then we should reset the properties panel.

[moreApi]

#463 will resolve the described issue but if I remember correctly sceneries volume manager currently doesnt handle deleting volumes correctly but that is not visible on the frontend.