A module for gathering code related to visibility automation and the like.
Contains DepGraphTools.py and TempoVis.py, to be withdrawn from
Part/AttachmentEditor.
Also: changes to TempoVis:
* keep TempoVis from from raising
(TempoVis is non-essential functionality, so breaking execution of
unsuspecting routines that use it seems like a bad idea.
* implement saving/restoring camera
(Unfortunately, I failed to implement nice animated restoration.)
* almost proper support of saving to file
(I still couldn't figure out a way to restore reference to document and
viewer. So, falling to using active ones instead (might cause bugs).)
