I have uploaded new builds here:
http://download.calibre-ebook.com/betas/
The changes:
1) Fix all reported bugs
2) Add preferences to customize keyboard shortcuts and the look of the editor (Edit->Preferences)
3) Integrate into calibre, replacing the old Tweak Book function, so that it now launches the new editor
4) Various small features, like replacing files, exporting files from the book to the disk, recently opened books, have only a single save book action, a save as option to save copies of the book and so on...
In my opinion the tool is now a useable replacement for Sigil. It is still missing some features, but those will come with time.
Please help test it, so that I can iron out the major remaining bugs and release it to the general public. I will post updated builds with bug fixes as bugs are reported.