Add generic cache for IDispatch objects

* src/dispcache.cpp, src/dispcache.h: New.
* src/gpgoladdin.cpp, src/gpgoladdin.h: Carry dispcache as
member.
* src/Makefile.am: Add new Files.

The cache shares the lifetime of the addin object and
can be used to keep IDispatch object around without having
to create them again and again.