Adjusted playable sides for LoW.

The changes are not final:
"Costly Revenge" and "Elvish Assasins" scenario sides need specific
adjustments for MP. E.g. gold, placement.
"Elves' Last Stand" and "The Chief Must Die" have different number of
sides than other scenarios in the same chapters.

In addition, mp::connect_engine's side controller set up has been
slightly adjusted.
Further adjustment for playable sides for LoW.

Since "The Chief Must Die" is not suitable for MP at this point, because
it has too few sides, it will be skipped in MP.
Improved "save_id" handling for SideWML (mp::connect).