Refresh tiles when map changes

[acrovato]

Only one map can be loaded at a time in the current version of the code. When the map is changed using the dropdown menu or via the settings page, the tiles that are already loaded remain. They need to be cleared and the map needs to be refreshed.
Note that the only way to change the map right now is to via the setting page and restart the app.

There are several issues and PR (619, 584) opened in the flutter_map repository that could solve this issue. If that does not work, consider to load all the maps using a list of layers.