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/cppmodelmanager.cpp

@@ -234,6 +234,11 @@ CppModelManager::CppModelManager(QObject *parent)
     , m_indexingSupporter(0)
     , m_enableGC(true)
 {
+    connect(this, SIGNAL(documentUpdated(CPlusPlus::Document::Ptr)),
+            this, SIGNAL(globalSnapshotChanged()));
+    connect(this, SIGNAL(aboutToRemoveFiles(QStringList)),
+            this, SIGNAL(globalSnapshotChanged()));
+
     m_findReferences = new CppFindReferences(this);
     m_indexerEnabled = qgetenv("QTCREATOR_NO_CODE_INDEXER").isNull();