Make a core/TinyMCE block type which integrates seamlessly with the gutenberg UI, and provides:

• backwards compatibility for legacy documents
• support for complex document types, e.g. multi column text with bullet lists in tables
• support for advanced features, e.g. font styles, text colours, indenting
• accessibility*
• mobile support**

*Accessibility
Use the gutenberg React UI components and extend them with accessibility features
Make new React UI accessible components for any advanced features not present in existing block types

**Mobile support
The TinyMCE block type could embrace TinyMCE mobile capability (rolled out later this year)

Related to discussions in #349 and #335. Note that this issue is orthogonal to #335 as 'core/TinyMCE' block type will deal with its own block type, not with broken, unknown or un-parseable block types (although it is possible that these block types could be channeled to TinyMCE block type for rendering)