Don't discard any OSM nodes with tags during loading

This affects a very small amount of nodes that are not currently rendered by Marble (e.g. various types of free-standing ticket vending machines). With this change those are also included in the generated o5m tiles, which matters for other users than Marble (and presumably Marble would want to show those properly eventually too).

This also require a minor change to Marble's style selection, as unknown nodes were shown with the default style (a yellow dot) so far, which is undesired I assume (but made it very easy to verify this is only a small amount in total).