QString::append: add (const QChar*, int len) overload

QString::append: add (const QChar*, int len) overload

Both insert and replace have this overload, so one reason to add it
to append(), too, is consistency. But I can also make good use of
this overload in the the new QStringList::join(QChar) overload, so
it's actually useful in its own right.

Change-Id: Iccd48f9cb84831399e4db7e3e78eba25c0ced30d
Reviewed-by: João Abecasis <joao.abecasis@nokia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>