FIX #77 Alow more control on unsharp settings

We can now choose between applying or not unsharp pass. We
also calculate the amount of unsharp based how much the resize
ratio is (between the  orignal and the target size). If we
apply it the next option are available:
    - Soft: Almost same as before
    - Hard: Double the amount of sharpness
    - 2.1 Soft: previous unsharp method, amount is fixed to 0.8
    - Fixed sharp: Same as artscript 1, we use a fixed value for
 everything.
    - None: No unsharp

NOTE: Unsharp is on by default for new settings.
FIX #57 Do not save thumbnails to disk

Artscript should avoid saving thumbnail files. Because it is
rare to process the same files several times we do not need to
store a collection of thumbnails for posterity.

Now artscript renders thumbnails on demand when a file is selected