Smooth Label Action

[cmhulbert]

Add an action to operate on label sources that applies a gaussian smoothing to the selected label. Some required features:

• [x] In-fill the portion of the label that was reduced with respect to the nearest adjacent labels.
• [x] show a 2D preview overlay in each of the visible ortho-slices while the user is specifying the desired sigma

[cmhulbert]

For a future iteration, some performance can be gained by determining which blocks need to be updated by applying the same smoothing function to a down-scaled image that represents a binary mask of label in the block grid space. (see #474 )