Function Node._input
Called when there is an input event. The input event propagates up through the node tree until a node consumes it.
It is only called if input processing is enabled, which is done automatically if this method is overridden, and can be toggled with setProcessInput.
To consume the input event and stop it propagating further to other nodes, SceneTree can be called.
For gameplay input, unhandledInput and unhandledKeyInput are usually a better fit as they allow the GUI to intercept the events first.