This change deals with issue #554.
If you enter a dive duration manually, the cell renderer cuts the seconds
away when the changes are saved. I added the helper "render_seconds_to_string"
as a counterpart to "parseDurationToSeconds". The helper keeps the seconds,
if not null. The rendering of the cell is done at two places in the code,
so I think it is cleaner to add a dedicated method for it.
Signed-off-by: Oliver Schwaneberg <oliver.schwaneberg@gmail.com>
eb38a64149