Use accessors for getting and setting PyGBoxed pointers

Add pyg_boxed_get_ptr and pyg_boxed_set_ptr macros for getting and setting
the boxed pointer field. This is preliminary cleanup work for supporting
fundamental types.

https://bugzilla.gnome.org/show_bug.cgi?id=631901
Use accessors for getting and setting PyGPointer fields

Add pyg_pointer_get_ptr and pyg_pointer_set_ptr macros for getting and
setting the pointer field. This is preliminary cleanup work for supporting
fundamental types.

https://bugzilla.gnome.org/show_bug.cgi?id=631901
Use accessors for getting and setting PyGParamSpec pointers

Add pyg_param_spec_get and pyg_param_spec_set macros for getting and
setting the GParamSpec pointer field held by the Python wrapper. This
is preliminary cleanup work for supporting fundamental types.

https://bugzilla.gnome.org/show_bug.cgi?id=631901