Deselect currently selected node by clicking on it

[YuriSizov]

Currently it's impossible to intentionally deselect any node, this can only happen automatically under certain circumstances. It is possible, however, to create disconnected parts of the same network by splitting existing paths, but due to the aforementioned limitation you cannot just go ahead and create new dangling nodes to start a new part.

The logic is already there, I just didn't implement the toggle for it. When clicking on the already selected node we should simply deselect it.