Do not allow the adition of a source that is already connected

If you try to connect a source that is already connected, it should
return and not create a duplicate of that source.

Signed-off-by: Artur Duque de Souza <asouza@kde.org>

declarativeimports/core/datasource.cpp

@@ -193,6 +193,10 @@ Plasma::Service *DataSource::serviceForSource(const QString &source)
 
 void DataSource::connectSource(const QString &source)
 {
+    if (m_connectedSources.contains(source)) {
+        return;
+    }
+
     m_newSources.append(source);
     m_connectedSources.append(source);
     m_changes |= SourcesChanged;