RFE: implement categorized logging for "QXcbConnection: XCB error"

currently, qxcbconnection.cpp contains:
...
qWarning("QXcbConnection: XCB error: %d (%s), sequence: %d, resource id: %d, major code: %d (%s), minor code: %d",
...
and in plasma5 users in practice end up seeing this in their logs... a lot. See also,
https://bugs.kde.org/show_bug.cgi?id=366421

It would be nice if this logging supported,
http://doc.qt.io/qt-5/qloggingcategory.html
so that these items could be more easily filtered.