Smarter placement of layers into the artboards that bound them

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

To improve upon this, we want:

- [x] Placing layers in correct artboard based on its location within the bounds of any artboard (#2110)
- [ ] Transferring layers between artboards when dragged from one to the other upon release
- [ ] When pasting from the clipboard, if no existing layers are selected, 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