Improve Wesnoth's engine to allow better transitions between scenarios

by Anja for Battle For Wesnoth

The idea is to clean up playcampaign.cpp and make loading/saving multiplayer campaigns more stable and less error prone. The code that handles transitions between scenarios should also be improved to make transfer of gold, units and recall lists easier for campaign designers. Other functionality, such as changes to difficulty between scenarios and interaction with GUI2 dialogs could also be added.