StringValues instead of StringLists (MCM menu)

- Untested!
- Updated MCM script to use the new version of the code, which writes
InitQuest, InitStage, UninstallQuest, UninstallStage as values on Quest
instead of on global lists.
- StringLists for RegisteredMods, InitMods and UninstallMods have been
removed.
- New helper function _GetModNameFromFormList() in MCM.psc.
- MCM.psc clears its MENU_OPTIONS arrays on menu close.