Use Q_EMIT instead of emit

In C++20 there are new classes that have member functions named emit().
c.f.:
https://lists.qt-project.org/pipermail/development/2020-February/038812.html
https://en.cppreference.com/w/cpp/io/basic_osyncstream/emit

A similar change was done in the Frameworks:
https://invent.kde.org/frameworks/kio/-/merge_requests/315

src/settings/ConfigurationDialog.cpp

@@ -108,14 +108,14 @@ void ConfigurationDialog::updateButtons()
     bool is_default = _manager->isDefault() && _groupManager->isDefault();
     setRestoreDefaultsButtonEnabled(!is_default);
 
-    emit widgetModified();
+    Q_EMIT widgetModified();
     onlyOnce = false;
 }
 
 void ConfigurationDialog::settingsChangedSlot()
 {
     updateButtons();
-    emit settingsChanged();
+    Q_EMIT settingsChanged();
 }
 
 void ConfigurationDialog::setApplyButtonEnabled(bool enabled)