API Change: Attach ImageViewer to Plugin after init.

Plugin is now added to the viewer using an inplace add on the viewer instead of on initialization of the plugin. This change means that operations requiring the viewer must be delayed until attach operation.

viewer_examples/plugins/canny.py

@@ -5,5 +5,5 @@ from skimage.viewer.plugins.canny import CannyPlugin
 
 image = data.camera()
 viewer = ImageViewer(image)
-CannyPlugin(viewer)
+viewer += CannyPlugin()
 viewer.show()

viewer_examples/plugins/lineprofile.py

@@ -5,5 +5,5 @@ from skimage.viewer.plugins.lineprofile import LineProfile
 
 image = data.camera()
 viewer = ImageViewer(image)
-LineProfile(viewer)
+viewer += LineProfile()
 viewer.show()