Improve Artboard tool selection behavior

[Keavon]

First, we want to always draw border overlays around all artboards to clarify what's clickable. Then, we need to default to one artboard always being selected (for discoverability, to avoid confusion about how you resize an artboard without any obvious handles shown). The user shouldn't be able to deselect all artboards to have 0 selected. For now, the artboard highest in the layer stack can be the default, and the tool saves which artboard was most recently selected (even if the tool has been switched away from, and then back to).