Don't show a pixmap for heading event

Some dive computers appear to have heading data in every sample. In order
to avoid a completely cluttered dive profile we no longer show a flag for
every heading event but instead show a basically transparent pixmap (which
is invisible to the user) that allows us to report the heading information
in the tooltip but leaves the profile uncluttered.

Fixes #586

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>