Support for HTML image grids/galleries.

* [ERROR_MESSAGES,SC] Exported CSS for errors.
* [DIMENSION,NEW] New module for calculating the dimensions of objects.
* [FLOAT_EXT,FIX] Integer outcome.
* [HTML_IMAGE,EXT] From support for images, to support for image boxes and image box grids. Added height and width parameters.
* [IMAGE,SC] New CSS file for images.
* [IMAGE_FILE,NEW] New module for image properties: added image_dimensions/3.
* [PAGE,SC] New CSS file for pages.