feat: Keyboard Layout applet: switch layout by mouse wheel

In parity with legacy layout Widget

FEATURE:
CCBUG: 390079

components/keyboardlayout/keyboardlayout.cpp

@@ -63,6 +63,11 @@ void KeyboardLayout::switchToNextLayout()
     if (mIface) mIface->switchToNextLayout();
 }
 
+void KeyboardLayout::switchToPreviousLayout()
+{
+    if (mIface) mIface->switchToPreviousLayout();
+}
+
 template<class T>
 void KeyboardLayout::requestDBusData(QDBusPendingReply<T> pendingReply, T &out, void (KeyboardLayout::*notify)())
 {

components/keyboardlayout/keyboardlayout.h

@@ -41,6 +41,7 @@ Q_SIGNALS:
 
 protected Q_SLOTS:
     void switchToNextLayout();
+    void switchToPreviousLayout();
 
 private:
     enum DBusData {LayoutDisplayName, LayoutLongName, Layouts};

components/keyboardlayout/org.kde.KeyboardLayouts.xml

@@ -9,6 +9,8 @@
     </signal>
     <method name="switchToNextLayout">
     </method>
+    <method name="switchToPreviousLayout">
+    </method>
     <method name="setLayout">
       <arg type="b" direction="out"/>
       <arg name="layout" type="s" direction="in"/>

components/workspace/KeyboardLayoutButton.qml

@@ -23,6 +23,18 @@ PlasmaComponents3.ToolButton {
 
     onClicked: layout.switchToNextLayout()
 
+    MouseArea {
+        anchors.fill: parent
+        acceptedButtons: Qt.NoButton
+        onWheel: {
+            if (wheel.angleDelta.y > 0) {
+                layout.switchToNextLayout()
+            } else {
+                layout.switchToPreviousLayout()
+            }
+        }
+    }
+
     KeyboardLayout {
         id: layout
     }