In the layer panel, drag layer(s) into a folder

[Keavon]

This extends upon #141 where we can now drag to rearrange layers in the layer panel and insert between other layers. But if a folder is empty, it isn't possible to insert between any children of that empty folder, so it's impossible to put it in that layer (without resorting to cut/paste). This should be altered to allow inserting the dragged layers into a folder itself, in addition to inserting between layers. That means the current behavior of putting the blue insertion marker line between the layers closest to the mouse's vertical location will need to be changed to check if the mouse is over a folder, and only show the blue line when close (say, 10px) from the top/bottom but when the mouse is in the center area of the folder's vertical area, outline the folder in blue.