improve functionality for save draft feature

In http://tracker.phpbb.com/browse/PHPBB3-11954 I gave some suggestions for how to improve the interface for the save draft feature, and I said I would post a second report with a suggestion to improve functionality. This is that second report.

I think it's unfortunate that posts can't be saved as drafts with attachments and any other attributes of a regular live post. In addition to losing the ability to save attachments with drafts, the user loses the ability to save polls and other things (like surveys for those using my tabulated survey mod) that can take a while to perfect and thus lead themselves to saving as drafts.

It seems to me that this is easily addressed by treating drafts as live posts (to be saved in the same table as regular posts) with the only difference being their visibility (this is effectively how I did it in my save full drafts mod for phpbb3, where I changed the post_approval column to post_status and one of the statuses indicated a draft). The corresponding code that includes posts or topics in viewtopic or viewforum or in any of the various post and topic counts would of course have to be tweaked so that drafts remain hidden, but that seems pretty straightforward.

As with the interface improvements I mentioned in my earlier report, my save full drafts mod implements this improved functionality for phpbb3, but it would be nice if I didn't need that mod for phpbb3.1 or 3.2.