Disable "download all files" after files have been deleted

[sssoleileraaa]

STR

1. Select a source with files that have not yet been downloaded
2. Delete the source conversation
3. Select the source menu

Expected

The "download all files" option to be disabled, since there are no longer files to download.

Actual

The "download all files" option is enabled, and if you select it then you get an error message in the status bar that the file could not be downloaded.

[sssoleileraaa]

@gonzalo-bulnes this is a followup issue for #1388

[gonzalo-bulnes]

For coordination: I'm looking into it.

[gonzalo-bulnes]

Reproduced the described behavior by following the STR 🍏

👉 Action plan:

• Refactor the DeleteConversationAction so that the actions interacts with the controller, and not the confirmation dialog (because that's harder to read and test). To that purpose, I've drafted a refresh of Refactor remove controller dependency from dialogs #1375 (I'll open a new PR and close that one instead of updating it - work in progress, a few action tests are missing).
  Edit: Refactor ensure actions hold the "commands" and confirmation dialogs implement Qt's API for dialogs #1447
• Ensure the interface of state.State contains a method to remove conversation files (by ConversationId to remove all the files at once I think, to be confirmed. We can make it file by file too.
  Edit: Confirmed, by ConversationId is convenient.)
• In the on_confirmation_dialog_accepted hook of DeleteConversationAction, add a command to update the state instance. With the refactoring above, this fix should be a one-liner, and should be enough to ensure that the DownloadConversation action is disabled when the conversation is deleted.
  Edit: Disable download when conversation was deleted #1448

👓 Longer term vision: this continues establishing a pattern where the updates of state.State that are caused by user actions will be triggered from the collection of Qt actions. Those updates are triggered along with the controller commands for now, until we start triggering the controller commands automatically when the state changes.