Clean the Utility::add_media() function

[alimuzzaman]

Update the function so that it can easily handle Retina and WebP sizes.

Also added new function Utility::get_path_and_url(). It's return gs_name and absolute path for all image size including full size.
It would be useful to use in compatibility, where we now have to write once instead of twice. Once for full size image and once for thumbnails.
Utility::add_media() also utilize this function now.