C++: Release more documents.

- fix memory leak in find-usages
- do not retain snapshot in search history
- when an editor is invisible for more than 2 minutes, release the
  backing snapshot

Retaining snapshots will retain their documents, and if done for too
long, the memory consumption might grow. This is especially the case
when switching to a different kit (Qt version): in that case, the new
versions of headers will be indexed, while the old ones stay around.

Task-number: QTCREATORBUG-5583
Task-number: QTCREATORBUG-7645
Task-number: QTCREATORBUG-9842

Change-Id: I045eda1565e0a3fa702baeffaab9c12662f90289
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>

src/plugins/cpptools/cppmodelmanagerinterface.h

@@ -279,6 +279,8 @@ signals:
     /// Other classes can use this to get notified when the \c ProjectExplorer has updated the parts.
     void projectPartsUpdated(ProjectExplorer::Project *project);
 
+    void globalSnapshotChanged();
+
 public slots:
     // Documented in source file.
     virtual QFuture<void> updateSourceFiles(const QStringList &sourceFiles,