Add framework for mime decrypt / verify

Add event handling for Application and MailItem
events and use the message incoming handler in
the before read event.

• src/comhelp.h: Removed again. Macros moved to mymapi.h where they are better placed.
• src/eventsink.h: (EVENT_SINK_DEFAULT_DTOR_CODE): Release / unadvise properly. (USE_INVOKE_ARGS): New. Used to avoid unused parameter warnings. (debug_oom, debug_oom_extra): Moved to util.h.
• src/eventsinks.h: New. Header to declare event sink install / detach functions.
• src/eventsinks.cpp: New. Event sinks for MailItem and Application.
• src/oomhelp.h: Define GUID's necessary for the Event handlers. (get_object_by_id): New. oomhelp API style wrapper around QueryInterface.
• src/oomhelp.cpp (get_object_by_id): New.
• src/util.h (log_oom, log_oom_extra): New. Logging helper.
• src/mymapi.h: Add Macros from comhelp. (IMAPISecureMessage): Declare COM interface.
• src/message.h: Add headers that were indirectly included in other usages.
• src/gpgoladdin.h (GpgolAddin): Add Application Event sink member.
• src/gpgoladdin.cpp (OnStartupComplete): Install Application Event sink.

This establishes the framework for proper MIME handling
currently mails are decrypted / verified but the message
content is not yet shown in Outlook.

The ApplicationEvent sink handles ItemLoad events (which should
happen once a Message is loaded into Outlook) and uses the event
to connect a MailItemEvent handler to the MailItem. The
MailItemEvent handler then handles the BeforeRead event and