[DRAFT] Improve paste support - RFC

Please have a look at this approach and leave comments (technical and design are all welcome) BUT DO NOT MERGE!

The current implementation always pastes new content as a single long string. This feels significantly unfinished. Adding support for paste with newlines greatly improves the paste experience, so it at least feels like a simple editor.

This patch is added two new features:
- Paste with new-line support (from plain text only)
- Experimental paste from HTML code

I don't intend to cover too much depth in the experiment HTML paste code, but longer term this will ideally include adoption of styling information and other such magics.