Make the settings dialog react to language changes

This covers the settings dialog itself and option pages based on
`UiFileBasedOptionPage`. Other option pages need to react to the
new `OptionPageWidget::retranslationRequired()` signal or handle
the `QEvent::LanguageChange` event which is now propageted to the
page's widget.

settingsdialog/optioncategory.cpp

@@ -1,6 +1,9 @@
 #include "./optioncategory.h"
 #include "./optionpage.h"
 
+#include <QCoreApplication>
+#include <QEvent>
+
 namespace QtUtilities {
 
 /*!
@@ -59,6 +62,20 @@ void OptionCategory::resetAllPages()
     }
 }
 
+/*!
+ * \brief Triggers retranslation of all pages.
+ * \remarks Has no effect if the pages don't react to the LanguageChange event.
+ */
+void OptionCategory::retranslateAllPages()
+{
+    auto event = QEvent(QEvent::LanguageChange);
+    for (auto *const page : m_pages) {
+        if (page->hasBeenShown()) {
+            QCoreApplication::sendEvent(page->widget(), &event);
+        }
+    }
+}
+
 /*!
  * \brief Returns whether the option category matches the specified \a
  * searchKeyWord.

settingsdialog/optioncategory.h

@@ -33,6 +33,7 @@ public:
     void assignPages(const QList<OptionPage *> &pages);
     bool applyAllPages();
     void resetAllPages();
+    void retranslateAllPages();
     bool matches(const QString &searchKeyWord) const;
     int currentIndex() const;
     void setCurrentIndex(int currentIndex);

settingsdialog/optionpage.cpp

@@ -99,6 +99,9 @@ bool OptionPageWidget::event(QEvent *event)
     case QEvent::PaletteChange:
         emit paletteChanged();
         break;
+    case QEvent::LanguageChange:
+        emit retranslationRequired();
+        break;
     default:;
     }
     return QWidget::event(event);

settingsdialog/optionpage.h

@@ -20,6 +20,7 @@ public:
 
 Q_SIGNALS:
     void paletteChanged();
+    void retranslationRequired();
 
 protected:
     bool event(QEvent *) override;
@@ -143,11 +144,14 @@ template <class UiClass> UiFileBasedOptionPage<UiClass>::~UiFileBasedOptionPage(
  */
 template <class UiClass> QWidget *UiFileBasedOptionPage<UiClass>::setupWidget()
 {
-    QWidget *widget = new OptionPageWidget();
+    auto *const widget = new OptionPageWidget();
     if (!m_ui) {
         m_ui.reset(new UiClass);
     }
     m_ui->setupUi(widget);
+    QObject::connect(widget, &OptionPageWidget::retranslationRequired, [this, widget] {
+        m_ui->retranslateUi(widget);
+    });
     return widget;
 }
 

settingsdialog/settingsdialog.cpp

@@ -337,6 +337,9 @@ bool SettingsDialog::event(QEvent *event)
     case QEvent::PaletteChange:
         setStyleSheet(dialogStyleForPalette(palette()));
         break;
+    case QEvent::LanguageChange:
+        m_ui->retranslateUi(this);
+        break;
     default:;
     }
     return res;