qt: Fix Qt 6 modifier key handling on macOS

Per QTBUG-69608 (https://bugreports.qt.io/browse/QTBUG-69608),
QKeyEvent::nativeVirtualKey() returns 0 on Qt 6 (and possibly some
versions of Qt 5) for keyboard modifier events. Despite being closed,
the bug doesn't appear to have been fixed, or perhaps was fixed and
then regressed again.

Detect this case and handle the events manually by keeping track of the
active modifier keys and using QKeyEvent::nativeModifiers() and the
platform-specific modifier values.

Caps Lock requires some special handling and doesn't get discrete
press/release events, at least with the Qt input system; see the code
for details. It's possible some other toggle modifiers like Scroll Lock
will need special handling along these lines too.

Fix #2211.

src/qt/qt_mainwindow.hpp

@@ -118,6 +118,11 @@ private:
     std::unique_ptr<MachineStatus> status;
     std::shared_ptr<MediaMenu> mm;
 
+#ifdef Q_OS_MACOS
+    uint32_t last_modifiers = 0;
+    void processMacKeyboardInput(bool down, const QKeyEvent* event);
+#endif
+
     /* If main window should send keyboard input */
     bool send_keyboard_input = true;
     bool shownonce = false;