Add OOM PropertyAccessor helper methods

* src/oomhelp.cpp (set_pa_variant): New. Set a property.
  (get_pa_int): New. Get an int property.
  (get_pa_variant): New. Get a generic property as variant.
  (get_pa_string): Refactored to use get_pa_variant.
* src/oomhelp.h: Add prototpyes and DASL defs.

This code might not be necessary as you mainly can access
MAPI directly. But it can be useful in case MAPI is out of
sync and gives weird values. This is added mostly as an
experiment for now as MAPI is usually easier to use.