Cannot spawn new monster groups during a scenario

[benknoble]

1. Sometimes you forgot a monster group
2. Sometimes you would rather play fog-of-war style where you don't know what's behind doors.

Supporting adding a new monster group mid-scenario (and handling its initiative correctly, which is tricky) would tackle both problems. For initiative, we probably need the "active group" from #21.

[benknoble]

Closed in 771146335ba7ef9746d60466163965f93fe1de32 and d9ac8d7209a8a9bd631eaca64d19b10a096bcb97 (v0.9~2 and v0.9~).