Use a proper @pyqtSlot for FilenamePrompt

In bleeding tests, we started to get a segfault on the second test in
tests/unit/mainwindow/test_prompt.py, with a stacktrace like:

    Thread 1 "python" received signal SIGSEGV, Segmentation fault.
    0x00007ffff7b55912 in _PyFunction_Vectorcall (func=<function at remote 0x7fffc4368ca0>, stack=0x7fffd74656a8, nargsf=<optimized out>, kwnames=0x0) at Objects/call.c:341
    341	   if (((PyCodeObject *)f->fc_code)->co_flags & CO_OPTIMIZED) {
    (gdb) bt
    #0  0x00007ffff7b55912 in _PyFunction_Vectorcall (func=<function at remote 0x7fffc4368ca0>, stack=0x7fffd74656a8, nargsf=<optimized out>, kwnames=0x0) at Objects/call.c:341
    #1  0x00007ffff4e0b3f1 in PyQtSlot::call(_object*, _object*) const (this=0x555556c759c0, args=('/tmp/pytest-of-florian/pytest-70/test_simple_completion_1_next_0/test',), callable=<function at remote 0x7fffc4368ca0>)
        at ../../qpy/QtCore/qpycore_pyqtslot.cpp:247
    #2  PyQtSlot::invoke(void**, _object*, void*, bool) const (this=0x555556c759c0, qargs=<optimized out>, qargs@entry=0x7fffffff86c0, self=<optimized out>, self@entry=0x0, result=result@entry=0x0, no_receiver_check=<optimized out>)
        at ../../qpy/QtCore/qpycore_pyqtslot.cpp:159
    #3  0x00007ffff4e12213 in PyQtSlot::invoke(void**, bool) const (no_receiver_check=<optimized out>, qargs=0x7fffffff86c0, this=<optimized out>) at ../../qpy/QtCore/qpycore_pyqtslot.cpp:78
    #4  PyQtSlotProxy::unislot(void**) (qargs=0x7fffffff86c0, this=0x555557193e20) at ../../qpy/QtCore/qpycore_pyqtslotproxy.cpp:205
    #5  PyQtSlotProxy::unislot(void**) (qargs=0x7fffffff86c0, this=0x555557193e20) at ../../qpy/QtCore/qpycore_pyqtslotproxy.cpp:186
    #6  PyQtSlotProxy::qt_metacall(QMetaObject::Call, int, void**) (this=0x555557193e20, _c=<optimized out>, _id=0, _a=0x7fffffff86c0) at ../../qpy/QtCore/qpycore_pyqtslotproxy.cpp:170
    #7  0x00007ffff48bd91d in doActivate<false>(QObject*, int, void**) (sender=0x555556b3d680, signal_index=28, argv=0x7fffffff86c0) at kernel/qobject.cpp:3945
    #8  0x00007fffeff96aca in QFileSystemModel::directoryLoaded(QString const&) (this=<optimized out>, _t1=<optimized out>) at .moc/moc_qfilesystemmodel.cpp:272
    #9  0x00007ffff48b0be0 in QObject::event(QEvent*) (this=this@entry=0x555556b3d680, e=e@entry=0x7fff0c004af0) at kernel/qobject.cpp:1347
    qutebrowser#10 0x00007fffeff962ab in QFileSystemModel::event(QEvent*) (this=this@entry=0x555556b3d680, event=event@entry=0x7fff0c004af0) at dialogs/qfilesystemmodel.cpp:1748
    qutebrowser#11 0x00007ffff070995c in sipQFileSystemModel::event(QEvent*) (this=0x555556b3d680, a0=0x7fff0c004af0) at /usr/src/debug/pyqt5/PyQt5-5.15.7/build/QtWidgets/sipQtWidgetsQFileSystemModel.cpp:376
    [...]
    qutebrowser#23 0x00007ffff4da94ce in meth_QCoreApplication_processEvents(PyObject*, PyObject*, PyObject*) (sipArgs=<optimized out>, sipKwds=0x0) at /usr/src/debug/pyqt5/PyQt5-5.15.7/build/QtCore/sipQtCoreQCoreApplication.cpp:590
    [...]

from pytest-qt in Python:

    Current thread 0x00007fb366207740 (most recent call first):
    File ".../pytestqt/plugin.py", line 209 in _process_events
    File ".../pytestqt/plugin.py", line 171 in pytest_runtest_setup
    [...]
    File ".../pytest/src/_pytest/runner.py", line 115 in pytest_runtest_protocol
    [...]
    File ".../pytest/src/_pytest/main.py", line 317 in pytest_cmdline_main
    [...]

Introduced by this change in pytest, which now deletes the temporary
directory after the test by default:

pytest-dev/pytest#10517

It sounds like something odd like the directory removal causing (Py)Qt
to call the lambda, but the underlying Python object already being gone
or something along those lines. With a proper Qt slot (@pyqtSlot),
PyQt seems to do the right thing, so let's just use that instead...

qutebrowser/mainwindow/prompt.py

@@ -760,9 +760,21 @@ def _init_fileview(self):
             self._file_view.setColumnHidden(col, True)
         # Nothing selected initially
         self._file_view.setCurrentIndex(QModelIndex())
-        # The model needs to be sorted so we get the correct first/last index
-        self._file_model.directoryLoaded.connect(
-            lambda: self._file_model.sort(0))
+
+        self._file_model.directoryLoaded.connect(self.on_directory_loaded)
+
+    @pyqtSlot()
+    def on_directory_loaded(self):
+        """Sort the model after a directory gets loaded.
+
+        The model needs to be sorted so we get the correct first/last index.
+
+        NOTE: This needs to be a proper @pystSlot() function, and not a lambda.
+        Otherwise, PyQt seems to fail to disconnect it immediately after the
+        object gets destroyed, and we get segfaults when deleting the directory
+        in unit tests.
+        """
+        self._file_model.sort(0)
 
     def accept(self, value=None, save=False):
         self._check_save_support(save)