Preserve keyboard focus when changing sorting order in the divelist

When the user changes the dive list sorting order via clicking
on different column titles, using gtk_widget_grab_focus() gives
keyboard focus back to the list itself (not staying on the column titles),
which gives a hint that the list itself has focus index of 0 and is
reset each time the widget receives this type of "initial" focus.

Acked-by: Miika Turkia <miika.turkia@gmail.com>
Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Indicate to the user a default divelist sorting order

Each time subsurface starts there is supposedly no sorting order
(or divelist column) specified by the UI, yet the actual column is '#'
(or dive number column), since its the *only* column which allows
trips to be visible. If the user selects a different sorting order
then he has no idea which column was the one who had the trips visible.

"Where did all those 'trip' things go?"
This can be a bit confusing...

Lets provide indication by calling gtk_tree_view_column_set_sort_indicator().
Also call gtk_tree_view_column_set_sort_order() to specify a descending
order.

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