This was a mistake, of course. Nested event loops don't need (or want)
independent server connections.
We initialize the connection early in GEventLoop for e.g. users that
want to get the size of a GDesktop before the connection has been
established.
Bug noticed by Andreas, introduced by me ;-)
As a consequence, move to use an explicit handshake() method rather than
calling virtuals from the constructor. This seemed to not bother
AClientConnection, but LibGUI crashes (rightfully) because of it.
