Empathy: Send prompt suggestions to server and display received prompts

Empathy: Send prompt suggestions to server and display received prompts

The received prompts can come in an initial list as part of the
"game-state" event sent when first connecting, and they can also come
in subsequent "prompt" events when a player submits a category
suggestion. These are all displayed as large buttons.

Clicking a button also sends a vote to the server, but received votes
are not yet displayed in any way.