Custom viewer
Better binning of categorical histograms
More progress and documentation on custom viewers
Custom widgets. Documentation + bball example
use six
Implement state save/restore for custom viewers
Call settings_changed at custom viewer creation time
Update Custom Viewer interface

Named argument introspection, subclass form, state special variable
Scope artist tracking to one layer
Tweaks to artist management
Link to viewer gallery in docs

[skip ci]
Merge pull request #444 from ChrisBeaumont/custom_widget

User defined data viewers