Make grid overlay naming more clear

Cell width/height might be misleading in the field of biology. Thanks to
Casey Schneider-Mizell for this observation. This commit changes the
labeling to "Grid width/height".
Fix wrong example site packages folder in simplified configuration

The abs_virtualenv_python_library_path setting should refer to the site-packages
folder of a virtualenv, not to the virtualenv folder itself. Thanks to Björn
Quast for bringing this to my attention.
SVG overlay: remove some unused variables
Tracing: store position of active node in unscaled stack coordinates

Storing the active tree node's position in scaled stack space
coordinates causes a problem when the zoom-level is changed: If one
doesn't re-select the active node its position isn't updated, which
would be required, because it is zoom-level dependent. To fix this, it
seems more cleaner to me to generally store the coordinates of the
active tree node in unscaled stack space rather than observing a stack's
scale change.

This change fixes #515, a 3D viewer issue which has been there for quite
a while: Centering the active node after changing the zoom-level would
result in a wrong position. That was because the 3D viewer expected the
active tree node's position to be in scaled stack space coordinates, but
these were not updated when the zoom-level was adjusted through the
slider.
Merge branch 'master' into multi-view-tracing
Fix calculation of active tree node's position in world coordinates

The active tree node now stores its position as unscaled stack space
coordinates. Therefore, there is no need for scaling anymore.