functionality to update gpx traces from files

[jamescr]

implements Upload gpx from file #36

[jamescr]

Thanks for the review, i applied the improvements to the code.

[westnordost]

Great, thank you! There is one thing left that is also relevant for your implementation in OSMTracker. You'll likely want to use try-with-resources so that the file handle is always closed also if an IO exception occurs:

try (InputStream in = new FileInputStream(file)) {
  // ...
} catch (IOException e) {
  // ...
}

[westnordost]

Nice, thank you! Will release a new version today.