Make "selection" area more flexible

[joto]

We should think about making the "Selection" area on the right more flexible, so that layers can do different things there.

@abrensch has done some changes there in his fork at http://brouter.de/osmoscope/ .

I am not sure what exactly we need here, but some kind of link back to the source of the layer seems like something other layers might need, too. Maybe we can find a generic way of doing this, for instance by allowing the layer description to define some links (based on an id field or so).