Adding a new ClientConnection class for wrapping wl_client

The ClientConnection is managed by Display. Whenever one tries to
get a ClientConnection for a wl_client* and it doesn't exist yet a
new one will be created and a clientConnected signal will be emitted.
Also there is a clientDisconnected signal.

ClientConnection provides access to pid, uid and gid. The idea is
to extend it to provide access to all the resources created for the
client.