MainWindow: maintain one window instance of the yearly statistics

This is a bit tricky because we are using a plain widget for
a window and don't have a class for it (req. more source files).
Also for the table model to update we need to create a new
YearlyStatisticsModel instance each time. At least, in that regard
we can re-create the model each time refreshDisplay() is called.

This patch adds a couple of private variables that are used
to manage the memory of the yearly statistics model and window
and also close that same window on MainWindow::closeEvent().

Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

qt-ui/mainwindow.h

@@ -156,6 +156,8 @@ private:
 	QAction *actionNextDive;
 	QAction *actionPreviousDive;
 	UserManual *helpView;
+	QTreeView *yearlyStats;
+	QAbstractItemModel *yearlyStatsModel;
 	CurrentState state;
 	QString filter();
 	static MainWindow *m_Instance;