divelist: prevent a crash for missing column width

The `static int defaultWidth[]` definition in divelistview.cpp
could potentially end up missing an element which can later result
in out-of-bounds access when iterating through the list of
columns and updating their widths.

Add a couple of methods in DiveTripModel for setting and getting
the widths and use those. The default values are now pre-set in a
QVector in the DiveTripModel() constructor.

Throw warnings if out-of-bounds columns are requested.

Reported-by: Gaetan Bisson <bisson@archlinux.org>
Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>

desktop-widgets/divelistview.h

@@ -70,6 +70,7 @@ private:
 	QModelIndex contextMenuIndex;
 	bool dontEmitDiveChangedSignal;
 	bool selectionSaved;
+	DiveTripModel *tripModel;
 
 	/* if dive_trip_t is null, there's no problem. */
 	QMultiHash<dive_trip_t *, int> selectedDives;