How should selection be applied to composite marks & mark overlays

[kanitw]

Composite marks (pending PR) and mark overlays (e.g., area with line and point overlay) generally produce multiple layers. It's unclear if specified selection should be applied to all layers or only some of them.

It's possible that it's better to apply to only some of them and the rule could depend on selection type.
For example, for line overlayed with points, "single" selection is probably applied to the overlayed points, but not the line (since we can't select individual points in the line). However, for boxplot, it's unclear what's the appropriate behavior esp. that some of the layers are ranged mark and/or aggregated and we don't have selection support for such cases yet.

Note: Since we've not thoroughly determine how to do this for all composite marks yet, our first release of composite marks will totally ignore selections as the behavior is not well defined. While the overlay will not copy/move the selection to the overlay layer. (It is better to not support them yet instead of adding partially broken support and then introduce breaking changes in the future.)

cc: @arvind, @domoritz, @jheer