Some improvements for smudge + hints + resources

You can toggle between smudge "liquify" (alt key) and "smooth" to blend colors, smooth details, distort reflections. Useful to create caricatures, simulate water and other interesting effects.

Comparing to others, Pencil2D's liquify is not the original algorythm but it's based on it, in order to be easily accessible and efficient to work with; it doesn't require an extra window. 

Regarding non-pixel-destructiveness and accuracy, future improvements are possible but they seem not urgent, not even important.