Implement support for multiple sort levels.
Implement support for multiple sort levels.

That is implement QMessageSortKey::operator+(),
 QMessageSortKey::operator+=() and support code.

This is not supported on WinCE, but is on Outlook MAPI, not sure about
 other MAPI implementations.

qtproject/qt-mobility,qtproject/qt-mobility,KDE/android-qt-mobility,enthought/qt-mobility,tmcguire/qt-mobility,enthought/qt-mobility,kaltsi/qt-mobility,qtproject/qt-mobility,KDE/android-qt-mobility,kaltsi/qt-mobility,enthought/qt-mobility,kaltsi/qt-mobility,tmcguire/qt-mobility,kaltsi/qt-mobility,qtproject/qt-mobility,enthought/qt-mobility,tmcguire/qt-mobility,enthought/qt-mobility,enthought/qt-mobility,qtproject/qt-mobility,kaltsi/qt-mobility,kaltsi/qt-mobility,qtproject/qt-mobility,KDE/android-qt-mobility,KDE/android-qt-mobility,tmcguire/qt-mobility,tmcguire/qt-mobility