selection: create global single_selected_trip() function

The DiveListView had a singleSelectedTrip function that
returns the selected trip if exactly one trip is selected.
This could be very slow if numerous non-trip items were
selected, because all the selection indices were back-
translated by the proxy model.

This could make selection changes very slow, because the
MainTab used said function to determine whether it should
show trip or dive data.. Indeed, with a 3500 dive test log,
when selecting all dives in tree mode, the updating of the
TabWidgets is sped up from 130 ms to 5 ms this commit.

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

desktop-widgets/divelistview.h

@@ -25,7 +25,6 @@ public:
 	~DiveListView();
 	void setSortOrder(int i, Qt::SortOrder order); // Call to set sort order
 	void reload(); // Call to reload model data
-	dive_trip *singleSelectedTrip();
 	static QString lastUsedImageDir();
 	static void updateLastUsedImageDir(const QString &s);
 	void loadImages();