Replace deprecated QLinkedList by std::list for #969
QLinkedList produces compiler 'deprecated' warnings when building 3rd party code since Qt 5.14.0. These fixes replace QLinkedList by std::list which is almost a drop-in replacement. Tested with Qt 5.12.9 and Qt 5.15.1 which are before and after the Qt 5.14.0 transition, so the changes should work for every Qt 5.
Not tested with Qt 6.