Add improvements to list fields (multiple selections)

The imagelist seemed like a really powerful field to me, but it felt clunky to use when I first tried it out. I've now made it possible to select multiple items, drag them around, and delete in bulk in both imagelists and filelists.

In imagelists you can magnific-ly view images by clicking on the thumbnail.

I think another useful feature (not in this) would be to select multiple items from the server like you can when uploading images.

It also fixes an error when trying to remove items in the list due to trying to iterate over a list that has been changed.