Feature/296 offline maps

[sgelb]

This PR implements basic support for offline maps and would close #296

Make it work

1. Download offline map data (maps and/or themes) from one of these map providers. I recommend OpenAndroMaps, as they also provide map themes.
2. Unzip if neccessary and copy into /sdcard/Android/data/fi.bitrite.android.ws/files/
3. You can now active offline maps and choose a theme (if installed) in the app settings.
4. All offline maps are activated automatically

Future ideas and plans (not for this PR)

• Option to choose map language (blocked until next release of OsmDroid, containing https://github.com/osmdroid/osmdroid/pull/1421)
• Option to manually activate/deactivate and sort offline maps
• Use deep links to install maps and themes from map providers
• Options for toggling map features like POIs etc
• Option to manually add more folders to search for map data, e.g. to reuse maps downloaded by Oruxmaps or Locus
• Add a help section into the app to explain how offline maps can be installed