Treat headings properly when importing Suunto logs

This patch will convert a heading bookmark to Subsurface format.
Suunto's bookmark uses "Heading: <degrees>°" format and was previously
set as the full event name. Now the resulting event will look like:

<event name="heading" value="333" time="0:58 min"/>

Signed-off-by: Miika Turkia <miika.turkia@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>