Pass Xcb errors through the native event filter

So far only the xcb plugin is able to handle xcb errors and there is no
way for an application to get access to the errors even if the app is
using xcb directly.

This means Qt is filtering out all xcb errors which are relevant to the
app and at the same time the xcb plugin is getting spammed with errors
it did not cause and which are logged with a qWarning.

By passing the error event through the native event filter an app can
filter out all errors it knows to have caused.

Change-Id: I158deee2e1c71630f2b1d77174f1091532851b3d
Reviewed-by: David Faure (KDE) <faure@kde.org>
Introduce QWheelEvent::Phase (Begin, Changed, Ended)

Some platforms (read: OS X) send wheel events without delta to indicate
that scrolling is about to start or has ended. Currently, Qt simply
ignores wheel events that have no delta. This change introduces a new
QWheelEvent attribute that specifies the phase, and makes it possible
to receive the special wheel events in started/ended phases. These
events are required for implementing correctly behaving transient
scrollbars.

Change-Id: Ib8ce0d9ce9be63b2ad60aa7b0aaa1f12ef6cad09
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>