rearrange cluster layout by dragging mouse

[anoop901]

Any changes to which clusters are in which layers should not violate the dependency constraint described in #92:

Clusters should be assigned to layers such that for any 2 clusters A and B where any block in cluster A depends (via line connection) on the root block of cluster B, cluster A is in a strictly lower layer than cluster B.

[anoop901]

The mouse can temporarily drag a block (along with blocks nested in it) to arbitrary locations. When the mouse is released, the layers field of the program should be updated, and the dragged block should snap to the location where the layout puts it with the new layers value.

The new value of layers should be determined by the location of the mouse when it is released.

• If the mouse is released within some close distance of an unconnected block input, the dragged block will become nested in that input.
• If the mouse is released in the space between layers, a new layer is created between those two layers, and the dragged block will be the only cluster in it. The dragged block will become a cluster root if it wasn't already.
• Similarly, if the mouse is released in the space above or below all layers, a new layer is created before or after all layers, respectively, and the dragged block will be the only cluster in it. The dragged block will become a cluster root if it wasn't already.
• If the mouse is released within a layer, the dragged block will be added to that layer. The index at which it is inserted is determined by the position of the mouse relative to the centers of the blocks in the layer. The dragged block will become a cluster root if it wasn't already.
• If the dragged block was originally a cluster root, that block is removed from its initial layer.

If updating layers as described here would violate the layer dependency constraint, then layers should remain unchanged instead.

[anoop901]

Also, if this update leaves any layers empty, they should disappear.