ENH: Add example of adding widgets to plugin

viewer_examples/plugins/canny_simple.py

@@ -0,0 +1,20 @@
+from skimage import data
+from skimage.filter import canny
+
+from skimage.viewer import ImageViewer
+from skimage.viewer.widgets import Slider
+from skimage.viewer.plugins.overlayplugin import OverlayPlugin
+
+
+image = data.camera()
+# Note: ImageViewer must be called before Plugin b/c it starts the event loop.
+viewer = ImageViewer(image)
+# You can create a UI for a filter just by passing a filter function...
+plugin = OverlayPlugin(image_filter=canny)
+# ... and adding widgets to adjust parameter values.
+plugin += Slider('sigma', 0, 5, update_on='release')
+plugin += Slider('low threshold', 0, 255, update_on='release')
+plugin += Slider('high threshold', 0, 255, update_on='release')
+# Finally, attach the plugin to the image viewer.
+viewer += plugin
+viewer.show()