Common cleanup: Cleaned up Common.h and simplified MsgHandler

In Common.h, I've only deleted defines/enums that were sitting there since the initial commit and weren't used anywhere at all. NOMINMAX is defined in CommonWindows so it looks safe to remove it here.

In MsgHandler, I've removed RegisterMsgAlertHandler and RegisterStringTranslator. Since users should see the alerts as rarely as possible, I don't think there will ever be a need to translate them.