QGeoTileCache: make generic for custom cache implementation

Currently the QGeoTileCache caches each tile in one file generating lots of files. I have two suggestions:

1.) Provide an interface for custom cache implementations or better
2.) Provide an option to either store the tiles in a SQL database (e.g. SQLite) or offer an overwriteable method which passes the tile data and the implementation can store the data as wanted (the same applies for reading cached tiles).