Added fixed size image generation + refactoring.

The generated images have now a different size from the original ones.
The file system proxy now integrates image resizing.
The dimensions of the generated images can't be specified at the moment.
The generated JSON is now different from the edition JSON :
    - the generated JSON specifies the width and height of images in px.
    - the edition JSON specifies constraints on width and height, but
      they are not required both (they can be set to null).
To reduce compilation time, the images are really deleted if they are
not used on any slides.
To avoid useless waiting time if the user wants to preview his
presentation twice without generating it, the 'preview' button doesn't
run the generation anymore (generation time has really increased due to
image resizing).
Some minor refactoring.