gui: write jquery module to submit forms in a hidden iframe

Right now, for example, the gpx upload function posts the form and causes the page to reload.
This is ugly because the map position will be lost, and well, it is just ugly.
Another consequence is that the map page has to know how to handle gpx uploads, which is not good. Every page should do just one thing and do it well.

In the future, the map should not move when uploading gpx files or editing a marker, but forms should be submitted in the background, and the map should be updated with the results.

Since this functionality can be reused in more than one place (gpx upload, marker editing), it makes sense to right a generic module for that. For example a new jquery module. It can be contributed back to jquery. Ain't that nice?