[SailfishOS] Implement "[name] is typing"

We already decrypt it and print a message to log::info!(...), this should be exposed in the UI. Additionally, we should send TypingMessage ourselves, making this an option in settings.

• UI work for the typing indicator (receiving)
• UI work for triggering the indicator send
• UI work for configuring whether to send TypingMessage
• Actually send TypingMessage
• Add a setting for it (affects both incoming and outgoing TypingMessages)
• Consider honouring the setting in debug logging, too