selection: replace selectedTrips() by singleSelectedTrip() function

To check wether the tab widgets should show the trip view, they called
the selectedTrips() function. The trip view was shown if that contained
only one trip. However, the selectedTrips() function was very slow,
because it has to query to core models.

Change the function to singleSelectedTrip(), which returns a trip
if there is exactly one trip selected. The function returns early
if there is more than one trip selected. This makes the select-all
case much faster.

There are two cases which are still very slow:
- List mode, because here all top-level items are queried.
- Dive log with many only top-level items.

Ultimately, we will have to cache the trip selection because
querying the model is too slow.

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

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