[applets/notifications] Use Header colors for notifications, if available

This brings the PlasmoidHeading used in the notification item into
visual consistency with other PlasmoidHeadings, once we merge
https://invent.kde.org/frameworks/plasma-framework/-/merge_requests/98.

When using a color scheme that lacks Header colors, there is no visual
change whatsoever.

applets/notifications/package/contents/ui/NotificationItem.qml

@@ -119,6 +119,9 @@ ColumnLayout {
         Layout.preferredWidth: notificationHeading.implicitWidth
         Layout.bottomMargin: -parent.spacing
 
+        PlasmaCore.ColorScope.colorGroup: PlasmaCore.Theme.HeaderColorGroup
+        PlasmaCore.ColorScope.inherit: false
+
         PlasmaCore.FrameSvgItem {
             imagePath: "widgets/plasmoidheading"
             prefix: "header"
@@ -128,6 +131,8 @@ ColumnLayout {
                 rightMargin: -margins.right
             }
             visible: !notificationItem.inHistory && fromCurrentTheme
+            colorGroup: parent.PlasmaCore.ColorScope.colorGroup
+            PlasmaCore.ColorScope.inherit: false
         }
 
         NotificationHeader {
@@ -138,6 +143,9 @@ ColumnLayout {
                 rightMargin: notificationItem.headingRightPadding
             }
 
+            PlasmaCore.ColorScope.colorGroup: parent.PlasmaCore.ColorScope.colorGroup
+            PlasmaCore.ColorScope.inherit: false
+
             inGroup: notificationItem.inGroup
 
             notificationType: notificationItem.notificationType