SqlExecutionArea: Support exporting result to CSV file

Add a button which allows the user to save the query results to a CSV
file.

Add some basic preparations for saving the result as a view.

src/sqlitetablemodel.h

@@ -26,6 +26,7 @@ public:
     bool removeRows(int row, int count, const QModelIndex& parent = QModelIndex());
 
     void setQuery(const QString& sQuery, bool dontClearHeaders = false);
+    QString query() const { return m_sQuery; }
     void setTable(const QString& table);
     void setChunkSize(size_t chunksize);
     void sort(int column, Qt::SortOrder order = Qt::AscendingOrder);