app: rework and fix the logic for copy-pasting multiple drawables.

There were a lot of incertainty of what should happen when we copy layers being
descendant of each other (i.e. when you select a group layer and some of its
children), then when you paste such data. So we sat down with Aryeom and tried
to come up with some consistent behavior which is somewhat expectable, but also
which would allow the most use-case.

Otherwise it was making very weird result when pasting the data, duplicating
some layers and whatnot, which was obviously a buggy behavior and never the
expected result.

We decided that if you select one leaf item, then even if you also selected a
parent item, it would be as though the parent was not selected. This is very
often what you expect anyway when you select a whole bunch of layers and would
work well if, say, you shift-click over many layers in sub-groups. Then you
wouldn't have to manually ctrl-click to unselect every group.

Then what if you were instead expecting to copy many groups? Then you could
shift-click the group arrow, closing all same-level groups. Once they are all
closed, you can shift-click the groups to only select group layers, not their
contents.

This way, both use cases are still quite doable easily with this default choice.