feature: Ability to add a street to another street

[kfarr]

As a user I want to progressively add "nodes" of a street one at a time. See (https://choochooworld.com/?DATA=DFFBHFIJB/0/D) for inspiration -- only one track can be placed at a time only at the end of the most recently placed track. This greatly simplifies placement for the user.

meta feature request ("job to be done"): how to add multiple streetmix streets from UI?