Pass number of turns for mp game using mp_game_settings.
Improve parameter passing between mp screens.
Move the initialization of the new level config to a different file.

This is initial step to unify the level initialization between
multiplayer_connect.cpp and playcampaign.cpp for multiplayer games.
Move the next level initialization for the non-host to a new function.
Move scenario & map generation to functions. Reduces code duplication.
Display mp::connect (temporary dummy) in between mp scenarios.

Currently, the screen between mp scenarios is displayed for local games
only.

The missing information to 'set_from_config()' and 'to_config()' were
added to have sufficient information to display mp::connect.
Apply changes from mp::connect in between MP campaign's scenarios.

The mp::connect is now shown only with '--debug' option. This avoids
multiple factions being applied to the same side over multiplayer
scenarios.