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/cpptoolseditorsupport.h

@@ -148,6 +148,9 @@ private slots:
     void updateEditor();
     void updateEditorNow();
 
+    void onCurrentEditorChanged();
+    void releaseResources();
+
 private:
     struct EditorUpdates {
         EditorUpdates()
@@ -160,7 +163,8 @@ private:
 
     enum {
         UpdateDocumentDefaultInterval = 150,
-        UpdateEditorInterval = 300
+        UpdateEditorInterval = 300,
+        EditorHiddenGCTimeout = 2 * 60 * 1000 // 2 minutes
     };
 
 private:
@@ -178,6 +182,9 @@ private:
     unsigned m_revision;
     QFuture<void> m_documentParser;
 
+    QTimer *m_editorGCTimer;
+    bool m_editorVisible;
+
     // content caching
     mutable QMutex m_cachedContentsLock;
     mutable QByteArray m_cachedContents;