Adapt to invalid begin iterator w/ QList/6 vs. QVector/5 on erase at begin

((stump branch in place of old commit))

Implementation of QList (replacing QVector via a typedef) in Qt6 when erasing at the begin of the list instead of memmoving data around just moves the internal pointer to the begin around. This breaks the assumption of the code written for QVector, caching the iterator to the start.