Partially offline maps

To make the app more usable, I've added offline maps for zoomed out maps. For closer and more detailed maps you will still need online connection. Once you zoom in, it will cache and temporarily work offline as well.

The intention was to make it faster, save your data usage and offload some of the traffic to OSM maps. After all, https://www.openstreetmap.org is why LetzMark app works and operates in free and privacy-driven mindset.