* Remove XFrame header set in \WebPage::no_cache : not this method responsability, was confusing :/
* Remove no_cache() calls when already set in page constructor (ajax_page mainly)
* Also calls everywhere the \WebPage::no_cache method instead of setting headers manually
- add a specific container for attachments list, upload button and #attachment_plugin hidden input is outside of it
- refactor code between abstract class and implementation, add some comments
- now refreshes only the attachment list instead of the whole content
