SCUMM: Add a "chained games manager"

This replaces the somewhat ugly use of the config manager to store
the chained games.
SCUMM: Add secret "easter_egg" config key

This makes it possible to override the detection of Maniac Mansion
when starting the Day of the Tentacle easter egg. There is no GUI
for setting this, no error handling, and setting it to Day of the
Tentacle itself is probably a bad idea...