FIX: final dimensions miss by a pixel
FIX: Final size will NEVER be bigger than the width or height given
by the user. Width and height are the limit.
FIX: convert uses same function to get sizes as the tree list, user
will view exactly waht the convert function will use
FIX: error if no resize selected
ADD: function to control quality depending on extension
FIX #25 : Make slider values change depending on format