Implement single-message deletion for messages in the current folder
This patch is only a step in the right direction, it's not a complete solution for performing operations on individual messages. It's going to take a lot more work to make the "Move Message to Trash" and "Delete Message" buttons work on any message in a conversation. However, being able to delete messages that reside in the current folder is still an improvement because:
It provides a way to do something that was previously impossible, even if you have to go into the folder that owns the message before you can delete it.
It provides a foundation and user interface for the message delete feature that can be extended and improved in the future.
It makes it more obvious that the "Delete Conversation" button only deletes messages from the conversation that are in the current folder. (This is an old bug, the same kind of architectural changes that are needed to make "Delete Message" work on any message would be needed to make "Delete Conversation" delete all messages in the conversation.)