Qt Location / Qml Maps: add support for custom (offline) tile sources

At the moment it seems only possible to use the built in map plugins (http://doc-snapshots.qt.io/qt5-5.5/qtlocation-index.html). It would be very useful to have a plugin (or interface) to add custom map sources. A use case is for instance an offline tile generator which provides tiles for the map. Common tile sizes are 256 and 512 pixels. iOS offers a comparable solution with the MKTileOverlay (since iOS 7): https://developer.apple.com/library/ios/documentation/MapKit/Reference/MKTileOverlay_class/index.html

Besides the fully customizable map source described above it would also be useful to have a generic map source for loading tiles from other web sources. Such a map plugin requires a server URL attribute with placeholders, for instance "http://tiles.osm.org/%x/%y/%z" where "%x/%y/%z" are placeholders for the tile coordinates.