Offline map tiles support in QtLocation

Currently, QtLocation plugins only support online map tiles.

Besides caching to improve tile visualization speed, there are a lot of scenarios where having access to pre-loaded tiles for a limited geographic area in offline mode is crucial. These include humanitarian mapping in/of areas with limited to no internet coverage, outdoors mobile applications, and custom tilesets.

There are two main strategies to store and access tiles offline: filesystem and sqlite (e.g. MBTiles: https://www.mapbox.com/guides/an-open-platform/#mbtiles).

A widespread convention for addressing offline tiles is the OSM, Google, Bing and Mapbox-compatible slippy map tilenames (http://wiki.openstreetmap.org/wiki/Slippy_map_tilenames), which is now also part of the WTMS standard (http://www.opengeospatial.org/standards/wmts).

This could be a self-standing plugin. Alternatively, it could add the functionality to one of the existing plugins, thus also allowing to cache tiles also from remote tile services.