Conversations: Maintain the unsaved conversation data when swapping between conversations

[CWolfs]

At the moment when you change conversations, any unsaved data on the conversation you were viewing is reset.

For this change:

• Keep the unsaved data of the conversations
• Restore the data if re-opening a conversation that has unsaved data
• Provide a visual indicator in the conversation tree of which conversations have unsaved data
• Warn the user when they try to close ConverseTek that unsaved data will be lost