cxx: Fix examples compilation.

cxx: Fix examples compilation.

Summary:
- Changed beta methods guards from CLASS_NAME_GUARD to
  EFL_BETA_API_SUPPORT to use the same scheme as C.
- Removed some includes to Efl_Ui.h from the examples. These were
  causing C's efl_part_get to not be generated due to EFL_PART_PROTECTED
  not being yet defined (it is defined in Elementary.hh, included
  afterwards). This was leading to Efl.Part.impl.hh trying to use a
  non-existent method.

Fixes T7716 partially (missing stringshare issue)

Test Plan: make examples

Reviewers: stefan_schmidt, felipealmeida, zmike

Reviewed By: zmike

Subscribers: cedric, #reviewers, #committers

Tags: #efl

Maniphest Tasks: T7716

Differential Revision: https://phab.enlightenment.org/D8284