Remove quit_session from main gui

It only makes sense when a game is actually running, so we make it part
of the session. This also helps to further distinguish between the main
GUI and the game's GUI.
Remove dependency on maingui from pause menu

Moved the save game function to the pausemenu and copied the code for
loading a game. The code is small and simple enough to be copied IMO,
better than having the dependency on the main gui.