[MapModel] Introduce types attributes and "Union"-Type for layers

[mbeckem]

Currently layer in the layer model do not have a attribute that can be used to determine the type of a layer (e.g. SimpleLayer, WMSLayer, ...). It would be helpful to introduce an attribute on the layer that specifies its type.

Additionally, it would be helpful to introduce a union type for layers that allows Typescript narrowing. Currently all layer types depend on a type, which is used in cases where multiple layer types (kinds) are allowed. However, it is not possible to narrow the layer type.

[mbeckem]

Note: the resulting union(s) should become the "official" Layer / Sublayer type. The current one should be renamed into a "base type" instead.