MacOS: Block shortcuts from overriding text input (#14869)

- It seems that on MacOS (vs other platforms), shortcuts for items in
  the application menu are given 'ultimate' priority, and will even take
  precedence over text inputs
- There is a mechanism in QT (I believe designed with mac in mind) to
  try to 'block' these shortcuts and send it to the focus instead. It's
  'Shortcut Override': https://wiki.qt.io/ShortcutOverride
- Initially I was going to only apply this check when it's a Command
  that overrides a known line-editing shortcut, but I figure it's
  simpler to just always apply it when editing text. I can't really
  imagine a user wanting to use an application shortcut while editing
  text, but if there's some compelling use-case for this then let me know
  and I'll add a further filter.

I'm quite optimistic that this won't have any ill-effects on other
platforms, but I'll need help from others to test this.

src/Gui/ShortcutManager.cpp

@@ -305,6 +305,26 @@ bool ShortcutManager::eventFilter(QObject* o, QEvent* ev)
         case QEvent::KeyPress:
             lastFocus = nullptr;
             break;
+        case QEvent::ShortcutOverride: {
+            auto kev = static_cast<QKeyEvent*>(ev);
+            if (!kev) {
+                break;
+            }
+            // don't process application shortcuts if we are editing a text widget
+            if (auto* focus = QApplication::focusWidget()) {
+                auto* maybeProxy = focus->focusProxy();
+                auto* focusOrProxy = maybeProxy ? maybeProxy : focus;
+
+                bool isFocusedWidgetTextInput = focusOrProxy->inherits("QLineEdit")
+                    || focusOrProxy->inherits("QTextEdit")
+                    || focusOrProxy->inherits("QPlainTextEdit");
+                if (isFocusedWidgetTextInput) {
+                    ev->accept();
+                    return true;
+                }
+            }
+            break;
+        }
         case QEvent::Shortcut:
             if (timeout > 0) {
                 auto sev = static_cast<QShortcutEvent*>(ev);