Client: Avoid processing of events when showing windows

Not tested yet, but applies cleanly. Lightly tested, no issues encountered.

BUG: 421700

The only time we want to dispatch events from the wayland socket is when
the application is waiting for external events. Doing so at any other
time will cause unpredictable behavior in client code.

This caused a crash downstream where we had outputs get altered whilst
itterating through outputs, which shouldn't happen.

There is no benefit to flushing here, it won't make anything appear
faster as we haven't attached the buffer yet.

Change-Id: Ie13eae4012dab96a93d8810f468d1343402b8c28
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Aleix Pol Gonzalez <aleixpol@kde.org>
(cherry picked from commit 46ed85a80b28d519cf5887bbdce55d1bf57886c3