eolian: allow tagging complete classes as BETA

eolian: allow tagging complete classes as BETA

Summary:
This allows using the @beta tag in classes, like this:
class @beta Efl.Foo extends Efl.Bar { ... }

This will surround the class definition in the .eo.h file with an
EFL_BETA_API_SUPPORT #define, equivalent to tag every method and
event with @beta.

Test Plan: Nothing changes since no class uses this tag yet

Reviewers: q66, bu5hm4n, zmike

Reviewed By: q66

Subscribers: cedric, #reviewers, #committers

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D7933