Support GeoPackage format

[fzadrazil]

Some users requested a geopackage data source for publishing layers.

[index-git]

Ogr2ogr support GeoPackage, so the format itself is fine. On the other hand, there can be many layers in one GeoPackage and also styles, which Layman does not support. One option is to support only GeoPackages with one layer, other is to implement layer extraction in client and upload layers one by one.

[jirik]

Another option is to change Layman to accept also GeoPackage with multiple layers and to publish all such layers. It would require some system changes in Layman, but it sounds more systematic to me than to extract GeoPackage layers at client and send them one by one to Layman.