Add a changed signal in Aggregation

Use it to change the find and candidate find object in CurrentDocumentFind:
this will allow to replace the IFindSupport object used.

Merge-request: 115
Reviewed-by: con <qtc-committer@nokia.com>

src/libs/aggregation/aggregate.h

@@ -74,6 +74,9 @@ public:
     static Aggregate *parentAggregate(QObject *obj);
     static QReadWriteLock &lock();
 
+signals:
+    void changed();
+
 private slots:
     void deleteSelf(QObject *obj);