Add support for WFS-T from QGIS Server

[jacobwod]

Hajk has drifted towards a GeoServer-centric approach recently, and great as GeoServer is, it's always good to give the users more options.

Adding WMS layers from QGIS is supported out of the box (though the admin experience could be improved). WFS Transactions however, require some code changes:

• Most notably, QGIS Server follows the WFS specification strictly and lacks support for outputFormat: "application/json" for describeFeatureType. We must handle the XML response in admin in order to be able to add layers.
• We must remove the geometry field somehow. The current approach is no good as QGIS Server will return the specific type of filed (e.g. gml:MultiLayerSurface), and not just Geometry as GeoServer seems to do. At the same time, we must keep the current support for the GeoServer style.
• On successful write, GeoServer always returned all types (totalInserted, totalDeleted and totalUpdated), while QGIS Server only returns the changed type. We can't expect all the properties to exist, so client needs some minor fixes too.

Overall, quite an addition, but I'll attempt to close this before the weekend.

[Hallbergs]

Nice job! I'm looking forward to try out QGIS Server after the summer!