About a month-or-so ago, I promised I would start work on selection tool for the image viewer. I finally got around to it. I ended up doing a pretty significant overhaul, but I think it was worth it.

See PR # 390

I'll copy my PR message below, just for completeness:

Major overhaul of tools for interacting with the scikit-image viewer.

* Add skimage.viewer.canvastools subpackage with line and rectangle selection tools
* Refactor line profile plugin to use line tool
* Fix viewer canvas
* Fix blitting behavior
* New plugins (ColorHistogram, Measure, Crop)

To get a feel for the changes, see new examples in `viewer_examples/plugins` located in the root directory of package.

Cheers,
-Tony