Smarter placement of layers into the artboards that bound them

[Keavon]

Currently, new layers are drawn, they are placed into the last selected artboard.

To improve upon this, we want:

• [ ] Placing layers in correct artboard based on its location within the bounds of any artboard
• [ ] Transferring layers between artboards when dragged from one to the other upon release
• [ ] When pasting from the clipboard, the content should go in the most recently selected (or drawn in/interacted with) artboard
  • [ ] When no layers are selected, clicking the New Folder button (in the Layers panel, next to the trash button) should place it in the most recently selected artboard

[Keavon]

Broken out from #1394

[nico-mayora]

I'm giving this a shot if that's all right.

[Keavon]

Sorry @nico-mayora, this is currently blocked by the ongoing with in #1712.

[nico-mayora]

@Keavon What about #944 then?

[Keavon]

That should be unblocked, I believe. As long as you're working with tool and visualization related code, not document format code.