Shallow Select mode should also display hover overlays for what a click and double-click will select

[Keavon]

Once #2392 is fixed, the Select tool's Shallow Select mode should correctly represent what clicking and double-clicking should do. Relative to the context of what is currently selected, single clicking should select a sibling (rather than anything that's a child of that sibling which the user clicks on). We want hovering to show a rectangular bounding box of what would get selected by clicking. Inside that is the item which will get selected by double-clicking (by "digging down" a level in the nesting). We also want that to show a bounding box at 50% opacity to indicate it would become selected by double-clicking. Instead of showing this 50% opacity bounding box, we should show the actual shape outline (the same one that we have right now on hover) at 50% opacity if double-clicking would select an object (rather than a group). And if that shape would get selected by single-clicking it, then we don't show any bounding boxes but we show the shape outline at 100% opacity just as we do today. In total, there will be up to three overlays shown, with only one at 100% opacity and up to two as bounding boxes.