Display of duration for which logging is enabled

I was replacing SportsTracker with your cool tool (since I don't use the functionality there to share or analyse tracks with their tools anyway). I basically only want to keep track of my sports activities for myself.

While trying that out, I noticed that a crucial feature missing for me was the duration for which the tracking was already going on. So I added it (rather quick&dirty). If you're interested, here are my changes! Feel free to use them in any future gpslogger version.

I also changed the display of the distance covered so far - just the integer kilometers seemed to crude a resolution for me when tracking e.g. a run; now always 3 digits after the comma will be shown (ideally that should probably be depending on the distance covered and the accuracy...)