Internal plugin references are not accessible after import

[mrbazzan]

On data import with djangocms-transfer, the Picture/Image plugin in the TextPlugin
does not render (It appears in the structure board but not in the actual plugin).

PS: The one that is encoded as <cms-plugin alt=\"Picture / Image - Image (274)\" title=\"Picture / Image - Image (274)\" id=\"274\"></cms-plugin>

Noticed here

[mrbazzan]

In djangocms-transfer, there is currently a _has_do_post_copy hook that determines whether a plugin's
do_post_copy method should be called (this method is responsible for changing the child plugin's id in the text's body).

TextPlugin defines do_post_copy method but it doesn't add the attribute hook, _has_do_post_copy which
allows djangocms-transfer to use the method. Instead of using the hook (which is undocumented I think), the
goal is to remove the hook entirely and instead directly invoke the post_copy method which is defined on all
models that inherit from CMSPlugin; thereby using documented behaviour.