Enable pure XBlocks to use children in edx-platform

The XModuleMixin is added to all XBlocks that are run in edx-platform (LMS or Studio) by edx-platform. So, the XBlock definition is pure, and the XModuleMixin provides the workarounds that the platform needs to use in order to work with the pure XBlock. Ideally, as we get rid of existing XModules, we can start removing XModuleMixin entirely.