Add support for the power and sleep buttons

This applies to Linux (and presumably OSX):

The power button was already handled but there was no mapping for it in keyboard.xml. The sleep button on the Nyxboard remote (and presumably others) was not handled. This commit addresses both issues.

All this SDL nonsense will be gone when we merge elupus and FernetMenta's changes, but for Frodo we still need to patch up keys not properly handled by SDL.