Settings widget: move grid settings creation into own function
Settings widget: make settings containers flow one after another
Settings widget: have label creation in separate function

This makes it more reusable.
Style change: make treenode module mor PEP8 compliant
Remove object tree related code

The object tree isn't used anymore. To avoid dead code, it's constructor and
references to the object tree itself are removed by this commit.
Add small style change
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.
Neuron name service: add basic settings infrastructure
Neuron name service: add basic name generation and client update

This commit extends the neuron name service to generate names for each
skeleton it manages. Currently, this is only the base name fallback and
no data is requested from the back-end, yet.
Neuron name service: add skeleton ID as display option
Neuron name service: add support for all and own annotation labeling
Selection table: let 'sort by name' use neuron name service
Neuron name service: support meta annotation based labeling
Graph widget: retrieve names from name service
Compartment graph plot: use name service for labeling
Connectivity widget: get neuron labels from name service
Connectivity widget: remember checkbox states on threshold change