Make the notification's popup title a bit smaller

Heading level 3 is just too big and barely fits the small notification
popup. I would like to avoid to make the notification wider and wider to
fit too big title, so I decided to make the title one level smaller

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

@@ -97,7 +97,7 @@ PlasmaCore.Dialog {
 
         PlasmaExtras.Heading {
             id: titleLabel
-            level: 3
+            level: 4
             height: theme.mSize(theme.defaultFont).height
             elide: Text.ElideRight
             anchors {