Implement a generic layer container for poi, tracks, routings, areas, etc

[dakk]

Since we're introducing always new geographical object, we need a more versatile container; an idea is to use a tree view for holding tracks, poi, routings, area to avoid (barrier) but we also need a new abstraction for data models. Something like QGis layer tree.

[dakk]

Every type of geom has a different number of columns, so this is not feasible