[Notifications] Restore item spacing

It got removed when the heading element was added.
A negative margin is added below the heading since it has its own spacing.

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

@@ -110,13 +110,14 @@ ColumnLayout {
     signal resumeJobClicked
     signal killJobClicked
 
-    spacing: 0
+    spacing: units.smallSpacing
 
     Item {
         id: headingElement
         Layout.fillWidth: true
         Layout.preferredHeight: notificationHeading.implicitHeight
         Layout.preferredWidth: notificationHeading.implicitWidth
+        Layout.bottomMargin: -parent.spacing
 
         PlasmaCore.FrameSvgItem {
             imagePath: "widgets/plasmoidheading"