Prosemirror binding

[mweidner037]

CRichText binding for the Prosemirror rich-text editor.

We will need to add support for block-based editing on top of CRichText (which is only meant for inline formatting). I'm not sure that we can support arbitrary Prosemirror schemas with reasonable semantics; we may need to support only "linear" block schemas (no nested blocks), or do something "good enough" for XML tree-type schemas.

For linear block schemas, I believe we can support a Notion-style block-move operation with the proper semantics (concurrent edits to a moved block are preserved), which would be neat.