QString: replace an inefficient QList with QVector

qt_section_chunk s larger than a pointer and wasn't marked as movable,
so holding it in a QList is horribly inefficient.

Change-Id: I32162b4960d32674a1f3b05bc24ac8813f16638d
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>