Allow walking the dive list with the cursor keys

Figure out what is our first selected element (in case we start out from a
multiple selection) and then move to the next logical element. So the code
traverses an expanded tree (from a trip 'down' to its first dive or 'up'
to the last dive of the previous trip - and similar from a first dive in a
trip 'up' to its trip and from a last dive in a trip 'down' to the next
trip.

This does not take 'shift-cursor-up/down' into account (i.e. manual
selection extension). Instead with just cursor up and down a single dive
(or single trip) is selected.

My guess is that the code will make someone's eyes bleed. Be warned.

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

qt-ui/mainwindow.h

@@ -9,6 +9,7 @@
 
 #include <QMainWindow>
 #include <QModelIndex>
+#include <QAction>
 
 class QSortFilterProxyModel;
 class DiveTripModel;
@@ -68,6 +69,10 @@ private Q_SLOTS:
 	void on_actionAboutSubsurface_triggered();
 	void on_actionUserManual_triggered();
 
+	/* keyboard actions */
+	void nextDive_triggered();
+	void previousDive_triggered();
+
 	void dive_selection_changed(const QItemSelection& newSelection,
 								const QItemSelection& oldSelection);
 
@@ -78,6 +83,9 @@ private:
 	Ui::MainWindow *ui;
 	DiveTripModel *model;
 	QSortFilterProxyModel *sortModel;
+	QAction *actionNextDive;
+	QAction *actionPreviousDive;
+
 	QString filter();
 	bool askSaveChanges();
 	void readSettings();