client: Remove the event thread

If the compositor sends events to us while the main thread is blocked
the socket notifier in the events thread would keep sending out the
activated() signal, but no events would actually be read until the
main thread starts to run again. That causes the event thread to keep
queueing new events, and so allocating memory, potentially forever.
This patch fixes the issue in maybe a bit radical way, that is by removing
the event thread. The socket notifier now runs in the main thread so it
will block if the events are not being read.
Nowadays there is no real reason to keep the event thread around, as every
thread that needs to receive wayland events can dispatch them on its own,
we don't need a central dispatcher thread anymore.

Change-Id: Ib7885e4b038b82719d78d193f465618a72cbe6af
Reviewed-by: Pier Luigi Fiorini <pierluigi.fiorini@gmail.com>