Allow "direct" grouping in Tree Window

[falko17]

Currently, the Tree Window can only group items together in a hierarchy-preserving way,
that is, elements that do not belong to a given group are still displayed as long as they have a descendant that is in the group.
We should also make it possible to disable this behavior such that only elements that are actually in a given group are included – we will call this "direct" grouping for the purposes of this issue.
As both behaviors are desirable in different situations, we should add a simple checkbox to the grouping menu that controls this.

As an example, in the first illustration below (assuming A and B are child nodes of R), the current behavior is illustrated: the relevant node hierarchy above the edges is included even though the reflexion state group is only applicable to edges.

Now, in the next illustration, we can see the desired behavior for this example when "direct" grouping is enabled.