Don't emit statusChanged if it hasn't changed

For some applets the status is bound to a complex expression that can be
re-evaluated a lot. This ends up causing a lot of unnecassary updates.

REVIEW: 126471

src/plasma/applet.cpp

@@ -453,6 +453,9 @@ Types::ItemStatus Applet::status() const
 
 void Applet::setStatus(const Types::ItemStatus status)
 {
+    if (status == d->itemStatus) {
+        return;
+    }
     d->itemStatus = status;
     emit statusChanged(status);
 }