Viewer: CheckBox Widget

Added a CheckBox widget for use with the viewer.  An interactive TV denoising example is provided where the checkbox can be used to control the 'multichannel' boolean.

This comment doesn't match the test: `2` -> `1`? Actually I don't think this comment actually adds any information (assuming that the values are 0 and 1; if it's 0 and 2 it is worth mentioning), so you can probably just remove it.