Add wayfinding sign variants and about text. Update exclusion dome artwo...

...rk. Add "Inception Train" as a 'secret' segment.

(P.S. @mwichary what I did was add a "secret: true" flag in the Inception Train segment. During creation of the toolbox, if this flag is encountered, it won't add it to the toolbox. This is a proposal for "secret" segments that aren't available in the common toolbox but only if someone remixes it from someone else's street, or potentially from another secret menu. @ahhrrr suggested the Konami Code.)