viewer / drawing - add funktion to unsharpen areas at a picture

[mokati]

base: xnview mp 1.8.3 / x64

problem:
sometimes in pictures it is necessary to make areas unsharpen (e.g. kids, faces, labels, or sensitive informations)

suggestion:
add a feature to the drawing-window with two options to make areas unsharpen:
- mosaic
- unsharpen