Input line

Input lines 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 line, they turn into a knot. Commands:
 * Input lines cannot have incoming pipes.
 * Input lines without an outgoing pipe can exist in a model, but this makes sense only as a temporary state.
 * Add incoming dendrite - the input line is extended with a new incoming dendrite with an input line at the tip. The original input line is transformed into a knot.
 * Select subtree
 * Trigger sound - open dialog for trigger sound
 * Stop on trigger on/off
 * Select nob/Deselect nob - add to/remove from selection


 * There is no explicit 'Delete' command for input lines. Deleting an input line would leave the outgoing pipe with an open end, which is not allowed. Instead you can delete the outpoing pipe. The input line at the start point of the pipe will disappear automatically.