i3bar: Handle DestroyNotify events

i3bar: Handle DestroyNotify events

Handle DestroyNotify events by removing the tray client from the tray
client list held in memory.

This change is intended to be part of the i3bar's implementation of the
XEmbed protocol. For more information, see:

<http://standards.freedesktop.org/xembed-spec/xembed-spec-latest.html>

According to the XEmbed protocol specification, this is one way for a
tray client to finish the protocol. After this event is received, i3bar
should have no more interaction with the tray client.