In the future we want to download trips into a distinct trip-table
instead of the global trip-table to allow for undo of import.
Therefore add a trip_table argument to DiveImportedModel::repopulate()
and a trip_table member to DiveImportedModel. To correctly set these,
add a DownloadThread::trips() function, which currently simply returns
the global trip table.
Finally, make "struct trip_table *" a Q_METATYPE, so that the corresponding
arguments can be passed from QML.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>