3D viewer: add view book marks

While a 3D viewer widget is open, one can now store different camera
positions. Stored views can be activated by selecting them in the drop
down list in the view tab. These views are not persistent.

Closes acardona/CATMAID#836
3D viewer: share views between instances

Moving the view storage into the prototype object allows easy sharing
between existing and new 3D viewer instances.