Stop holding passwords in memory when opened in details view

[jimporter]

Currently, when we load an item's details (in the doorhanger or the manager), we fetch all the item details, including the password. This means that the password is stored unencrypted in memory whenever the details are opened. We could make this more secure by only holding onto the password when it's actually visible, as well as using an async method to fetch the password for copying (as we do in the quick-copy mode).