Describe frame content concept and implement base hierarchy.

Frame contents should be:

• replaceable
• non-defined
• unsupported
• various kinds(text, audio, video, other)
  

The user should be able to insert a frame without a content and add it afterward or can change the content when he decides. The content can be unsupported if it was created with non-present plug-in for example. The frame and the content of the frame can be considered as separated parts.

Make base implementation of the frame contents hierarchy. (for example it should be implemented with composition of the frame content in the frame)