Track properties: split track at marker.

Track properties: split track at marker.

I often find myself in the need of trimming tracks generated from my GPS.
This is how I can do with this new added feature:

- Bring up the track properties dialog (right click on the track and select
  "Properties".
- Click on the one of the graphs to select where I want to split. I'd check
  the center cross on the map to make sure it is at the point I want to split.
  I don't mind to do few more click to get exactly to where I want.
- Click "Split at Marker" to split the track.
- I might have to do all over again to trim the other side of the track.
  (one day, multiple markers can make things much easier).

Signed-off-by: Quy Tonthat <qtonthat@gmail.com>