Exclude news account folders from list of target folders for Move and Copy

When you invoke Quick Folder Move's Move or Copy commands, it lists target folders from all accounts, regardless whether they are mail accounts, news accounts, or feed accounts.

For Move and Copy, it makes no sense for Quick Folder Move to list any news account folders, as these are read-only. It is impossible to move or copy a message to such a folder, even with Thunderbird's built-in move/copy commands. If you try to copy or move a message into a news account with Quick Folder Move, it silently fails.

Quick Folder Move should never show any news accounts or their folders in its Move/Copy lists.

Start at Quick Folder Move's Move and Copy command handling and inspect how target folders are collected across accounts. Reproduce the issue with a news account, then verify that neither the account nor its folders appears in either list while other account folders remain available.

Written by the indexing model from the issue text.