Added new fields to the metadata block of the saved GPX file.
There is a new combobox on the save page that lets user select the type of track/excercise. The content of the combobox is written to tag <type> of the metadata block.
Before saving the file, the track is analysed and the following properties are also calculated and added to tags in the metadata block:
  - duration [s]
  - distance [m]
  - avg_speed [m/s]
  - max_speed [m/s]