Provide generic region of interest definition

Many workflows demand the definition of a region of interest (ROI).

In order to support this in a generic way, I suggest to let the user load arbitrary vector data into individual map layers and then promote one of those layers to the region of interest.

Therefore we have to give the user an overview over the currently loaded layers, e.g. via ol-layerswitcher. In order to select a layer as the current ROI, we add a button to each (vector) layer, that (on click) promotes the corresponding geometry of the layer to be the user selected ROI.