[applets/digital clock] Restore scroll-to-change-displayed-timezone

After the rest of the UI changes made to the applet and the time zone
config page, I don't think users will be confused by the differences
between the timezone displayed in the clock, and the systemwide local
timezone anymore. Therefore, it should be safe enough to bring back the
wheely time feature.

BUG: 431977

applets/digital-clock/package/contents/config/main.xml

@@ -48,6 +48,10 @@
     <entry name="lastSelectedTimezone" type="String">
       <label>When multiple time zones are configured, this is the one shown on widget restore. Typically the system's current timezone.</label>
       <default>Local</default>
+    </entry>
+        <entry name="wheelChangesTimezone" type="Bool">
+      <label>Whether the mouse wheel switches between the timezones configured in selectedTimeZones.</label>
+      <default>false</default>
     </entry>
     <entry name="displayTimezoneAsCode" type="Bool">
       <label>Whether the timezone is displayed as a code i.e. "GMT" or full text i.e. "London".</label>

applets/digital-clock/package/contents/ui/DigitalClock.qml

@@ -423,7 +423,44 @@ Item {
 
     MouseArea {
         anchors.fill: parent
+
+        property int wheelDelta: 0
+
         onClicked: plasmoid.expanded = !plasmoid.expanded
+
+        onWheel: {
+            if (!plasmoid.configuration.wheelChangesTimezone) {
+                return;
+            }
+
+            var delta = wheel.angleDelta.y || wheel.angleDelta.x
+            var newIndex = main.tzIndex;
+            wheelDelta += delta;
+            // magic number 120 for common "one click"
+            // See: https://doc.qt.io/qt-5/qml-qtquick-wheelevent.html#angleDelta-prop
+            while (wheelDelta >= 120) {
+                wheelDelta -= 120;
+                newIndex--;
+            }
+            while (wheelDelta <= -120) {
+                wheelDelta += 120;
+                newIndex++;
+            }
+
+            if (newIndex >= plasmoid.configuration.selectedTimeZones.length) {
+                newIndex = 0;
+            } else if (newIndex < 0) {
+                newIndex = plasmoid.configuration.selectedTimeZones.length - 1;
+            }
+
+            if (newIndex !== main.tzIndex) {
+                plasmoid.configuration.lastSelectedTimezone = plasmoid.configuration.selectedTimeZones[newIndex];
+                main.tzIndex = newIndex;
+
+                dataSource.dataChanged();
+                setupLabels();
+            }
+        }
     }
 
    /*

applets/digital-clock/package/contents/ui/configTimeZones.qml

@@ -32,6 +32,8 @@ ColumnLayout {
     id: timeZonesPage
 
     property alias cfg_selectedTimeZones: timeZones.selectedTimeZones
+    property alias cfg_wheelChangesTimezone: enableWheelCheckBox.checked
+
 
     TimeZoneModel {
 
@@ -47,7 +49,7 @@ ColumnLayout {
 
     QQC2.ScrollView {
         Layout.fillWidth: true
-        Layout.preferredHeight: Kirigami.Units.gridUnit * 19
+        Layout.preferredHeight: Kirigami.Units.gridUnit * 16
         Component.onCompleted: background.visible = true // enable border
 
         ListView {
@@ -143,10 +145,20 @@ ColumnLayout {
         onClicked: timezoneSheet.open()
     }
 
+    QQC2.CheckBox {
+        id: enableWheelCheckBox
+        configuredTimezoneList.count > 1
+        Layout.fillWidth: true
+        Layout.topMargin: Kirigami.Units.largeSpacing * 2
+        text: i18n("Switch displayed time zone by scrolling over clock applet")
+    }
+
     QQC2.Label {
         visible: configuredTimezoneList.count > 1
         Layout.fillWidth: true
-        Layout.margins: Kirigami.Units.largeSpacing * 2
+        Layout.topMargin: Kirigami.Units.largeSpacing * 2
+        Layout.leftMargin: Kirigami.Units.largeSpacing * 2
+        Layout.rightMargin: Kirigami.Units.largeSpacing * 2
         text: i18n("Note that using a different time zone for the clock does not change the systemwide local time zone. When you travel, switch the local time zone instead.")
         wrapMode: Text.Wrap
     }