Improved GPX bounding box handling

I had added the option to upload a GPX track in the MapOSMatic instance a little while ago, but using that was a bit tricky. This has just been improved a little big.

So far you had to manually select the map area to render before adding a GPX track which could be a bit tricky.

Now it is possible to select one of three options:

• keep original bounding box (default, as before)
• extend (merge) the bounding box to fully contain the GPX track, too
• replace the selected bounding box with that of the GPX track completely