Features and references with missing ID parts are detected and reported

[l-laura]

The mapget model implementation currently creates features from a tile without validating if the feature ID fields are set. This will lead to problems with implementing e.g. jump-to-feature down the line, where a feature reference must contain all the information to be resolved to a feature.

To fix the issue, the mapget model should check that the ID fields are set, before adding a feature or feature reference to the feature layer.