FRED should autosave more often

[Goober5000]

FRED will autosave Backup.001 through Backup.009 mission files after the following actions:

fredrender.cpp(1210):FREDDoc_ptr->autosave("level object");
fredrender.cpp(1234):FREDDoc_ptr->autosave("level objects");
fredrender.cpp(2195):FREDDoc_ptr->autosave("align object");
fredrender.cpp(2220):FREDDoc_ptr->autosave("align objects");
fredview.cpp(1075):FREDDoc_ptr->autosave("object move");
fredview.cpp(1224):FREDDoc_ptr->autosave("Editing");
fredview.cpp(2243):FREDDoc_ptr->autosave("form wing");
fredview.cpp(2308):FREDDoc_ptr->autosave("wing disband");
fredview.cpp(4320):FREDDoc_ptr->autosave("object delete");
fredview.cpp(4330):FREDDoc_ptr->autosave("clone marked");
fredview.cpp(4340):FREDDoc_ptr->autosave("wing delete");

It would be helpful to autosave more often. In particular, FRED should autosave whenever the Event Editor is closed.