Rewrite readSettings to do what it should have done from the start

This function was supposed to take the default_prefs into account but
clearly wasn't.

Now it should be much more readable and maintainable.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

qt-ui/mainwindow.h

@@ -82,6 +82,7 @@ private Q_SLOTS:
 	void on_actionUserManual_triggered();
 
 	void current_dive_changed(int divenr);
+	void initialUiSetup();
 
 protected:
 	void closeEvent(QCloseEvent *);