Minimize warmup message

Warmup message is minimized to the top left corner after 10 seconds, if no other player is present. The message is pure white as blinking is pretty annoying. This fixes #1040.

This is how it looks like:
![screenshot_2013-09-03_23-38-46](https://f.cloud.github.com/assets/529264/1076355/2f752948-14e1-11e3-9a80-6a5d7a07e46e.png)