Major rewrite of the application

New features:

• [#5] Adding undo & redo
• [#8] Supports other map sources than GeoPortail (see https://map2gpx.eu)
• Supports importing tracks using KML and GeoJSON formats (additionally to GPX)
• Supports importing tracks via a URL
• Supports exporting tracks to GeoJSON format

Improvements:

• Better controls for drawing tracks
• Better popups to display data along the track
• Much much faster to load, compute tracks and getting stats