keyboard_layout: Add shortcut to toggle last-used keyboard layout

This is a companion to
c01b583e53
that enables the above functionality on Wayland. For Xorg, the
functionality is implemented in plasma-desktop in the above commit.

src/keyboard_layout.cpp

@@ -49,6 +49,16 @@ void KeyboardLayout::init()
 
     connect(switchKeyboardAction, &QAction::triggered, this, &KeyboardLayout::switchToNextLayout);
 
+    QAction *switchLastUsedKeyboardAction = new QAction(this);
+    switchLastUsedKeyboardAction->setObjectName(QStringLiteral("Switch to Last-Used Keyboard Layout"));
+    switchLastUsedKeyboardAction->setProperty("componentName", QStringLiteral("KDE Keyboard Layout Switcher"));
+    switchLastUsedKeyboardAction->setProperty("componentDisplayName", i18n("Keyboard Layout Switcher"));
+    const QKeySequence sequenceLastUsed = QKeySequence(Qt::META | Qt::ALT | Qt::Key_L);
+    KGlobalAccel::self()->setDefaultShortcut(switchLastUsedKeyboardAction, QList<QKeySequence>({sequenceLastUsed}));
+    KGlobalAccel::self()->setShortcut(switchLastUsedKeyboardAction, QList<QKeySequence>({sequenceLastUsed}));
+
+    connect(switchLastUsedKeyboardAction, &QAction::triggered, this, &KeyboardLayout::switchToLastUsedLayout);
+    
     QDBusConnection::sessionBus().connect(QString(),
                                           QStringLiteral("/Layouts"),
                                           QStringLiteral("org.kde.keyboard"),
@@ -99,6 +109,16 @@ void KeyboardLayout::switchToLayout(xkb_layout_index_t index)
     checkLayoutChange(previousLayout);
 }
 
+void KeyboardLayout::switchToLastUsedLayout()
+{
+    const quint32 count = m_xkb->numberOfLayouts();
+    if (!m_lastUsedLayout.has_value() || *m_lastUsedLayout >= count) {
+        switchToPreviousLayout();
+    } else {
+        switchToLayout(*m_lastUsedLayout);
+    }
+}
+
 void KeyboardLayout::reconfigure()
 {
     if (m_configGroup.isValid()) {
@@ -154,6 +174,7 @@ void KeyboardLayout::checkLayoutChange(uint previousLayout)
     // We need OSD if current layout deviates from any of these
     const uint currentLayout = m_xkb->currentLayout();
     if (m_layout != currentLayout || previousLayout != currentLayout) {
+        m_lastUsedLayout = std::optional<uint>{previousLayout};
         m_layout = currentLayout;
         notifyLayoutChange();
         Q_EMIT layoutChanged(currentLayout);

src/keyboard_layout.h

@@ -12,6 +12,7 @@
 #include <QObject>
 #include <QVector>
 #include <memory>
+#include <optional>
 
 #include <KConfigGroup>
 #include <KSharedConfig>
@@ -43,6 +44,7 @@ public:
     void checkLayoutChange(uint previousLayout);
     void switchToNextLayout();
     void switchToPreviousLayout();
+    void switchToLastUsedLayout();
     void resetLayout();
 
 Q_SIGNALS:
@@ -63,6 +65,7 @@ private:
     QVector<QAction *> m_layoutShortcuts;
     KeyboardLayoutDBusInterface *m_dbusInterface = nullptr;
     std::unique_ptr<KeyboardLayoutSwitching::Policy> m_policy;
+    std::optional<uint> m_lastUsedLayout;
 };
 
 class KeyboardLayoutDBusInterface : public QObject