cms.utils.copy_plugins.copy_plugins_to(): reworked, new code reuse

Now returns the list of new plugins, rather than new and old plugins zipped.
Behavior was retained, since the return value was only used once, in a len().
If the recipient wants the previous result, they can rezip the result and the
submitted parameter. This allowed the reuse of copy_plugins_to() and
encapsulated the mutating old_parent_cache parameter.

cms.utils.placeholder.MLNGPlaceholderActions.copy():
now reuses copy_plugins_to()