Divelist: fix a crash when updating the preferences

Hitting apply in the preferences dialog causes a QList assert.
This led to DiveListView::reloadHeaderActions(), where
we have an out of range access.

Patch makes the column count match the header action count.

Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>