Improve Format::Class docs.
Define more granular ordering of builtin tags.

Add more extensive documentation for Tag#html_position.
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.
Define member sections rendering in Tag classes.

The titles and ordering of the sections is now defined within the Tag
class of each member type.  Ordering works similarly to the ordering
of #to_html method calls.

Titles are no more hardcoded in Render::Class.
Generalize BaseType to detect any of the registered member types.

Custom member types can now finally be detected.
Remove hard-coded property-like members from signature rendering.

Take into account all the registered property-like tags when deciding
whether to render in method or property style.