PWA support, minor stuff

• Paths' widgets are now hidden only by one spoiler.
• Added drawing limits to the rectangle layer.
• Implemented correct parallel and meridian line length calculations.
• Grid labels now can be localized.
• Implemented Leaflet.Geodesic support for Leaflet.Draw. It'll be moved to a separate library.
• Line layer and paths' connections are geodesic lines now.
• Done some useless work for line layer. Most of the code will be removed when new Leaflet.Geodesic version will be released.
• Added PWA support, yay!
• Changed logo.