Integrate topics in the chat window for group chats

[Robby]

For IRC and XMPP, topics should be displayed in the chat window.

• jas8522: "I kinda like how Colloquy does it with a little bar at the top of the webkit view - perhaps would could do something similar..."

• brainball: "Pidgin uses something similar to Colloquy, a simple bar along the top of the view with the option to click on it to edit the topic."

This ticket consolidates #7788 and #10376.

[zacw]

Removing 'Group Chat' component.