FRED has an autosave feature but it wasn't invoked in too many places. This adds autosaving whenever a major dialog is closed, unless it was closed by hitting Cancel. NB: Autosave is not yet implemented in qtFRED.
Also, `addvariabledlg` incorrectly referred to `OnOk` instead of `OnOK` and `CMessageEditorDlg` incorrectly referred to `OnOK` instead of `OnOk`, so this fixes those and distinguishes `OnOk` by renaming it `OnButtonOk` to be consistent with the event editor and other dialogs.
This introduces a new mechanism for handling bit flags. Instead of using an int (or two, or three) to write bits into, this uses the std::bitset in a wrapper class that accepts an enum class as input. This means that certain classes of errors we've had a few of in the engine, where a coder used the wrong set of flags or checked them against the wrong variable (SF_STEALTH vs SIF_STEALTH, for example) can now be detected at a compiler level rather than not at all.
Goober5000