edje_cc: add 'skip_namespace_validation' keyword

edje_cc: add 'skip_namespace_validation' keyword

this pairs with the newly-added -N option to denote groups in edc files
which should not be checked for correct namespacing

the option is useful for cases such as music_control.edc,
where a group has been shipping for years with API signals like
"btn,clicked" which cannot be changed since they are used in an external
application and compatibility must be maintained

the documentation for this option explicitly states that it must only be used
inside an #ifdef SKIP_NAMESPACE_VALIDATION block, allowing this keyword to
be easily removed at a later point

ref T7072

@feature

Differential Revision: https://phab.enlightenment.org/D6388