Introduce member type categories.

Divide members into two broad categories:

 - method_like
 - property_like

The @param Tag merger is now set up for all method_like members - so
all such members could have parameters.

The Default and Type tags are set up for all property_like members -
so all such members will have the :default and :type fields processed
and merged.

This should lay ground for the future with user-defined member types.