Crouton in a tab

Still a few issues to iron up, more testing, and a few missing features, but the general idea is there.

Known issues:
 - Sometimes the extension does not start with focus (you need to click)
 - Touch events
 - IME events
 - "Error target STRING not available" sometimes appears during switch (croutonclip/cycle?)
 - Limit number of resolution changes when resizing (it's slow because we send far too many "R" requests)
 - A leftover FIXME in the code, we should force courtoncycle instead of removing files
 - Move the websocket functions into a common file
 - Errors when loading NaCl module are not reported
Yes, probably a good idea not to start the DM if the display cannot be connected, anyway.

I did it this way to save a few seconds. In practice it does not make much of a difference (at least for xfce...)