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/cppeditor/cppfunctiondecldeflink.h

@@ -67,7 +67,7 @@ private slots:
 private:
     QTextCursor m_scannedSelection;
     QTextCursor m_nameSelection;
-    QFutureWatcher<QSharedPointer<FunctionDeclDefLink> > m_watcher;
+    QScopedPointer<QFutureWatcher<QSharedPointer<FunctionDeclDefLink> > > m_watcher;
 };
 
 class FunctionDeclDefLink
@@ -75,8 +75,6 @@ class FunctionDeclDefLink
     Q_DECLARE_TR_FUNCTIONS(CppEditor::Internal::FunctionDeclDefLink)
     Q_DISABLE_COPY(FunctionDeclDefLink)
 public:
-    ~FunctionDeclDefLink();
-
     class Marker {};
 
     bool isValid() const;