Prototype Outlook 14 support

This adds a prototype "Save and decrypt" action for attachments.
The main advancement is that gpgOl can now interact with
Outlook 14 using the Ribbon extension interface.

* src/gpgoladdin.cpp, src/gpgoladdin.h: New. Classes implementing
the COM Objects to interact with outlook.
* src/gpgoladdin.cpp, src/gpgol.def (DllGetClassObject),
(DllCanUnloadNow): New entry points into the library.
* src/olflange.cpp (DllRegisterServer, DllUnregisterServer):
Register / Unregister gpgOl as an Outlook Addin.
(GpgolExt): Use VERSION macro.
(install_sinks, install_forms): Expose for other classes.
* src/olflange.h: Move GUID and some string definitions into
header. Add function declarations.

• In outlook 14 the exchange extension interface was removed. Now we have to use the IDT_EXTENSIBILITY2 interface to interact with outlook (or other office applications). Similarly the context menu events in the OOM have been removed now we must use the Ribbon Extensibility Interface to extend context Menus. This is also the only way to avoid having all added buttons placed in a generic "Addins" tab and gives us some more flexibility how we can modify the UI. There are still some fixmes and todos in there but basically it works. Not tested with older outlook versions where the Exchange interface also works. In that case it might be best to check the Version in OnConnection and disable all functionality in case it is < 14.