Cleanup: unify selection signals

For historic reasons, there where three distinct signals concerning
dive-selection from the undo-machinery:
1) divesSelected: sent newly selected dives
2) currentDiveChanged: sent if the current dive changed
3) selectionChanged: sent at the end of a command if either the selection
   or the current dive changed

Since now the undo-commands do a full reset of the selection, merge these
three signals into a single signal.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

desktop-widgets/divelistview.h

@@ -42,6 +42,8 @@ public:
 	QList<dive_trip *> selectedTrips();
 	static QString lastUsedImageDir();
 	static void updateLastUsedImageDir(const QString &s);
+signals:
+	void divesSelected();
 public
 slots:
 	void toggleColumnVisibilityByIndex();