[Regression] QSortFilterProxyModel::setSourceModel() fails to set a new source model

The latest change in QSortFilterProxyModel::setSourceModel() caused a regression bug which breaks QSortFilterProxyModel.

The offending commmit: http://code.qt.io/cgit/qt/qtbase.git/patch/?id=1c0fcbc887459d8963088309e83303eb1a7d2db0

The "destroyed()" signal of the old model is never disconnected. So after you set a new model and delete the old one, QSortFilterProxyModel:: is reset by the destroyed() signal emitted by the old model and the proxy model is broken.

The root cause is you called `_q_sourceModelDestroyed()` manually before calling base class `QAbstractProxyModel::setSourceModel()`, which resets d->model to the static empty model object. So in `QAbstractProxyModel::setSourceModel()`, the following line stop working since it disconnects from the static empty model instead of from the old model object. So the old model is never disconnected.

disconnect(d->model, SIGNAL(destroyed()), this, SLOT(_q_sourceModelDestroyed()));

You can reproduce the bug easily with the test case I attached.

#include <QSortFilterProxyModel>
#include <QStandardItemModel>
#include <assert.h>

int main(int argc, char** argv) {
    auto proxyModel = new QSortFilterProxyModel();
    auto model1 = new QStandardItemModel();
    auto model2 = new QStandardItemModel();
    proxyModel->setSourceModel(model1);
    proxyModel->setSourceModel(model2);
    delete model1;
    assert(proxyModel->sourceModel() == model2);   // the result becomes null while it should be model2
    return 0;
}

proxymodel_bug.cpp

Related discussions in LXQt community: https://github.com/lxqt/libfm-qt/pull/197