cxx: Fix some warnings from -Wextra

cxx: Fix some warnings from -Wextra

Summary:
- As we don't have C++17's [[fallthrough]], rely on GCC'd detection
  of fallthrough comments.

  See https://developers.redhat.com/blog/2017/03/10/wimplicit-fallthrough-in-gcc-7/

- Replaced some copy constructors with typecast operators.

  Previously, a constructor with remove_const/remove_cv were used
  to allow const iterators to be constructed from non-const iterators.
  This had the side effect of making these constructors actual copy
  constructors for non const lists. As we did not define other
  special constructors/operators, the rule of 5 were violated for these
  cases.

  This commit replaces these constructors with actual typecast operators
  that promote non const iterators to their const counterparts.

- Cast a Eina_Bool/bool narrowing conversion

- Add a missing break statement from D10050

Reviewers: zmike, brunobelo, felipealmeida

Reviewed By: brunobelo

Subscribers: cedric, #reviewers, #committers

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D9934