Display confirmation dialog for killing unresponsive clients

When a client is unresponsive, we should put up a dialog asking the user if they'd like the client to be killed. This will need a new protocol object in the private compositor UI protocol.

xfwm4 does this when attempting to close a window: it sends _NET_WM_PING after sending the WM_CLOSE_WINDOW message, and if the client doesn't ACK the ping within 3 seconds, the dialog is shown. Support for the ping/kill mechanism is being added as a part of #16.

xdg_shell also has a ping mechanism, which smithay exposes but appears not to use internally; I could use it to send pings and wait for pongs, and display the unresponsive dialog if they don't respond in time.