Make GpgolAddin class a singleton
* src/gpgoladdin.cpp (GpgolAddin::get_instance): New. * src/gpgoladdin.h (GpgolAddin::get_instance): Declare. (GpgolAddin::get_application): Expose application reference.
This makes the Application reference we get on connection
globally accessible which is useful as it can serve as an
entry point into OOM.