Add initial implementation of Kopete plugin, using DBUS.

Sending files does not work, at least for me, with MSN (I don't know why) or
Facebook (expected because FB does not allow file transfers; OTOH Kopete
erroneusly reports FB contacts as file_transfer-enabled).

Let's see if Kopete developers can help with file transfer; I'll try to use
libkopete-dev if I don't get a solution. In the future, this plugin should
use Telepathy KDE.