Input line

Input neurons are nobs which have no natural counterpart in the human neural system. They are only tools for the construction of a model. When plugged into an output neuron, they turn into regular neurons. Commands:
 * Input neurons cannot have incoming pipes.
 * Input neurons can have only one outgoing pipe, just like ordinary neurons.
 * Input neurons without an outgoing pipe can exist in a model, but this makes sense only as a temporary state.
 * Add incoming dendrite - the input neuron is extended with a new incoming dendrite with an input neuron at the tip. The original input neuron is transformed into a regular neuron.
 * Select subtree
 * Trigger sound - open dialog for trigger sound
 * Stop on trigger on/off
 * Select nob/Deselect nob - add to/remove from selection
 * Delete - delete the input neuron