clients: make main event loop more robust

This makes the toytoolkit event loop more robust. Now it uses the
canonical wl_display_prepare_read(), read() and cancel().

Also, it allows functions that run before the event loop to create
Wayland queues and dispatch events related to such queue. Before our
changes, this would cause issues, because of how the loop was written
and also because dispatch_queue() reads the display fd and queue them on
the appropriate event queues, it doesn't read only the events related to
the queue we are interested.

Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>