QSortFilterProxyModel could use a stack to keep track of changes to the source model, similar in concept to what we do with managing persistent model indexes in QAbstractItemModel.

I.e. in source_items_about_to_be_removed(), the items to be removed shouldn't be removed straight away, but rather be pushed on the stack; then in source_items_removed(), the stack is popped and we update both source_to_proxy and proxy_to_source at the same time, so they're in sync.

This should fix the bug in task 167273 in the right(tm) way, from the QSortFilterProxyModel perspective.