GPS::GpsmanData::Tk: smaller fixes
data update
refactored gps_data_viewer into own module GPS::GpsmanData::TkViewer
GPS::GpsmanData: new method sort_waypoints_by_time

With test.
GPS::GpsmanData: new method get_sorted_waypoints_by_time

This one is like sort_waypoints_by_time, but non-destructive and just
returns the waypoint list.

With test.
mark_layer moved to bbbike main program

Also fixed the mark_layer call in GPS::GpsmanData::TkViewer, which did
not work since the recent refactoring.
bbd2kml: use <Point> for one-coordinate features
strassen-kml.t: test case for one-coordinate features
Merge branch 'kml-points'
gpsman viewer: support for associated Waypoints in GPS::GpsmanData::Tk
GPS::GpsmanData::TkViewer: optionally include associated wpt file