Integrate create_engine into sp campaign selection

After campaign selection, the mp codepath is followed starting with
enter_configure_mode().

Conflicts:
	src/game_controller.cpp
	src/multiplayer.cpp
[scenario] tags now convert to [multiplayer] in sp

Also, moved the "Custom" mp side to be loaded outside of the MULTIPLAYER define.
A blank placeholder era has been created for sp.
Prevent reload if MULTIPLAYER not defined
Removed blank era from MULTIPLAYER load
Kludge to skip multiplayer_configure in sp
Integrated parameter initializer into mp_configure
Clean up new_campaign code
Implemented multiplayer_configure_engine
Add force_lock_settings logic to configure engine

Also, changed use_map_settings_default to check for force_lock_settings. Now set_default_values also initializes use_map_settings to default, so we won't have to do it manually.
Fixed bool connect_canceled not being initialized
Implemented gui-less mp_connect for sp

Also, made sides with blank controllers default to ai in sp.
Hard-code era for sp

For some reason, sp was attempting to load era_default sometimes.
Changed the way play_game is called for sp

Previously, in sp, play_game was called from both enter_connect_mode and goto_campaign (via launch_game). I've edited enter_connect_mode to return true instead of calling play_game, so launch_game is used instead. (This contains some code relevent to sp campaigns, but still uses the mp-codepath-modified saved_game (state_) object.)
Changed sp defaults for fog and village gold.

In order to match the settings of old sp, fog now defaults to false and village_gold now defaults to 1.
Added hybrid campaigns to list of sp campaigns
Change the way defaults are loaded in mp_configure

Previously, parameters were all set to defaults, then gui widgets were initialized from the parameters.
Now, widgets are initialized directly to the defaults, and parameters are mostly left untouched until get_parameters() is called.
Also, use_map_settings is now initialized to default in configure_engine constructor.
Moved difficulty selection code to create_engine
Clean up some code that was commented out
Rename create_engine in sp for clarity
Rename data_mutable() to data()