Dynamic beans: support cross namespace inheritance

Quoting PCentgraf from HSTTWO-4676:

E. It is likely that we will ship a distribution for SaaS with a number of predefined document types and component implementations out-of-the-box. This will likely lead to more cross-namespace extension situations. It is also valuable to know what will need to change in the document type editor to support this and what problems it will cause for our dynamic content beans implementation.

Issue HSTTWO-4676 contains examples of such namespaces, specifically see types "myproject:extendedHippoMirror" and "myproject:extendedThirdPartyType". This issue is about:

1. Discuss cross namespace requirements and analyse what is realistic and what not, and what needs we will have on SaaS
2. Based on the output from above, change implementation to support cross namespace features as required