Move all dimensions operations to Picture::Transformations

All point and string arguments can tage the "x20" or "20x" arguments.
Points get 0 interpolated, height or width get the width or height of
the crop_size or of the original image.

Upscaling is not allowed unless upscale is set to true at the end of method
calls.
NICE! No more double imagemagick calling. Great \o/