create next_scenario_settings member of end_level_data

it is read and written during serialization
next_scenario_settings is read from [endlevel] WML tag
carryover WML is applied at start of scenario by playcampaign.cpp
add and implement next_scenario_append
merge the "allow_new_game" field before test for mp ui
Merge pull request #98 from cbeck88/carryover_WML

feature: [endlevel] may specify carryover WML merged/appended into next scenario