Always delay stage 2 shutdown, not only with prompts

If we shut down qutebrowser directly, in some rare situations, we seem
to get into some trouble related to a QWidget trying to call into a
Q(Common|StyleSheet)Style at shutdown (checking the style hint
SH_Widget_ShareActivation, to figure out the palette to use when the
window is hidden, or somesuch?).

With the recent changes for #7504, those crashes suddenly started
happening much more often. After staring at the related changes and gdb
for over a day, I still have no idea what actually causes them and why
it's crashing - it smells like some kind of PyQt/Qt cleanup order bug...

However, what I *did* find out (after a lot of painful debugging and
messing around with things) is that delaying the shutdown done from
:quit seems to reliably get rid of the issue. I don't know *why*, but
I haven't seen it happen anymore with the latest changes...

Fixes #5385 and #5124, both speculatively.

Closes #7504 since this was the last change required to make it all
work.

qutebrowser/mainwindow/prompt.py

@@ -131,20 +131,12 @@ class PromptQueue(QObject):
         """Cancel all blocking questions.
 
         Quits and removes all running event loops.
-
-        Return:
-            True if loops needed to be aborted,
-            False otherwise.
         """
-        log.prompt.debug("Shutting down with loops {}".format(self._loops))
+        log.prompt.debug(f"Shutting down with loops {self._loops}")
         self._shutting_down = True
-        if self._loops:
-            for loop in self._loops:
-                loop.quit()
-                loop.deleteLater()
-            return True
-        else:
-            return False
+        for loop in self._loops:
+            loop.quit()
+            loop.deleteLater()
 
     @pyqtSlot(usertypes.Question, bool)
     def ask_question(self, question, blocking):

qutebrowser/misc/quitter.py

@@ -221,21 +221,18 @@ class Quitter(QObject):
             status, session))
 
         sessions.shutdown(session, last_window=last_window)
+        prompt.prompt_queue.shutdown()
 
-        if prompt.prompt_queue.shutdown():
-            # If shutdown was called while we were asking a question, we're in
-            # a still sub-eventloop (which gets quit now) and not in the main
-            # one.
-            # This means we need to defer the real shutdown to when we're back
-            # in the real main event loop, or we'll get a segfault.
-            log.destroy.debug("Deferring real shutdown because question was "
-                              "active.")
-            QTimer.singleShot(0, functools.partial(self._shutdown_2, status,
-                                                   is_restart=is_restart))
-        else:
-            # If we have no questions to shut down, we are already in the real
-            # event loop, so we can shut down immediately.
-            self._shutdown_2(status, is_restart=is_restart)
+        # If shutdown was called while we were asking a question, we're in
+        # a still sub-eventloop (which gets quit now) and not in the main
+        # one.
+        # But there's also other situations where it's problematic to shut down
+        # immediately (e.g. when we're just starting up).
+        # This means we need to defer the real shutdown to when we're back
+        # in the real main event loop, or we'll get a segfault.
+        log.destroy.debug("Deferring shutdown stage 2")
+        QTimer.singleShot(
+            0, functools.partial(self._shutdown_2, status, is_restart=is_restart))
 
     def _shutdown_2(self, status: int, is_restart: bool) -> None:
         """Second stage of shutdown."""