Show UnsavedDocumentsDialog when closing window or quitting application

so user can save and or discard docs more efficiently.

pglab/querytool/SqlDocumentRegistry.h

@@ -0,0 +1,27 @@
+#ifndef SQLDOCUMENTREGISTRY_H
+#define SQLDOCUMENTREGISTRY_H
+
+#include <QList>
+#include <memory>
+#include <vector>
+
+class SqlDocument;
+
+/** Keeps track of all SQL documents that are still in use, without
+ * extending their lifetime. A document lives as long as at least one
+ * query tool holds a shared pointer to it.
+ */
+class SqlDocumentRegistry {
+public:
+	void add(const std::shared_ptr<SqlDocument> &document);
+
+	/// All documents that are still alive.
+	QList<std::shared_ptr<SqlDocument>> documents() const;
+	/// All documents that are alive and have unsaved changes.
+	QList<std::shared_ptr<SqlDocument>> modifiedDocuments() const;
+
+private:
+	std::vector<std::weak_ptr<SqlDocument>> m_documents;
+};
+
+#endif // SQLDOCUMENTREGISTRY_H