Fixed inherited child plugins not being loaded and rendered correctly

Inherited child plugins weren't being rendered because their child plugins weren't computed. The commit associated with this PR fixes this issue. See the commit message for more details.

This has been tested only on my local django-cms installation, and I didn't find any issues caused by this change there. That said, I hadn't fiddled with django-cms internals before, so this change may do something stupid and/or unintended. Please check and review before merging.