The form to create/copy an item should include a folder picker

At the moment, new items are placed at the top level of the project. The form should include a field to choose a folder for the item to go in.

Start by locating the form used to create or copy items and the code that places new items at the project top level. Trace how folders are represented and selected there. Done means the form offers a folder picker and newly created or copied items are placed in the selected folder.

Written by the indexing model from the issue text.