Describe frame content concept and implement base hierarchy.[[BR]]

Frame contents should be: 
 * replaceable 
 * non-defined
 * unsupported
 * various kinds(text, audio, video, other)[[BR]]

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.[[BR]]

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