edje: Revert build fix and make "internal" eo public

edje: Revert build fix and make "internal" eo public

Those classes are simply badly named, and I've come to realize since
that part class definitions are not internal, but instead they are
part of the public API. They are required for documentation, casting,
etc... I will rename them to "part" instead of "internal" later.

Since the headers are included in Edje_Eo.h not installing them
broke the build of any external app. Sorry for the mess!

This reverts 665856027d85802a29ede6f005674a8294df60fe

Ref T5306