Preview: Add button to remove received files via the UI
Going through a file manager/terminal is very inefficient, but most importantly seems to be pointless, as Gajim might automatically re-download the file on another occasion. The easiest way to tell Gajim to put a kind of tombstone on the file would be by going through the UI (but bonus points for watching the downloads directory for removals).
The feature would be best modeled after Conversations:
- a 'Remove file' context menu item for file-messages;
- a visual 'File was removed' tombstone;
- ability to re-download manually;
- the message history is left intact, as the point of the feature is keeping downloads tidy (e.g., for storage space oder Ordnung), not redacting past conversations.
AFAIK, Conversations also watches the directory for removals, but not entirely sure.