Merge single- and multi-player codepaths

This PR contains a fully functional implementation of sp campaigns using the mp create, configure, and connect engines. There should be no noticeable change to gameplay. Code for launching singleplayer games (handling the engines) is contained in singleplayer.*pp, similarly to multiplayer.
Once this is merged, we can begin to introduce some mp-specific options (eras, observers, etc) to sp. My plan is to make these available through an "Advanced Settings" dialog accessible from the current sp campaign selection dialog.
It's better to bring enter_configure_mode into its old scope as you are not using it for SP anymore.