MapView shall allow tiles to be cached

[Kittyfisto]

Currently tiles are always downloaded from their source server. They shall also be cached locally (if desired) both in-memory and on disk. This should be achieved using IsabelDb (which can do both over the same interface).

This is also a good point to move all map related controls into a different project and nuget package (e.g. Metrolib.Map) due to having more dependencies.