[Refactor] Use Leaflet

This PR contains a huge refactor to use Leaflet instead of the Google Maps API to...

* use OpenStreetMap instead of Google Maps as default, but with the ability to switch back to Google Maps
* add further layers (like, for example, airports and navaids) without pain

In this PR, I also fixed a weird design issue with hover effects on fullscreen sidebar buttons and I increased the version number to 1.4 because I think it's great. ;)