Flickering lights, buttons now saved in savegames

The flickering light effect of sectors of type 17 (from DOOM II) is now
saved in savegames, so now the flickering of those sectors will resume
when a savegame is loaded. See
http://doomwiki.org/wiki/Sector_type_17_is_disabled_after_loading_a_saved_game.
Active buttons are now also saved.