This is an unexpected and very nice development - I mean potentially including of Sigil functionality in Calibre. It belongs of course to category "let's do something else".
Speaking for myself only: I like Sigil as it is now, a standalone dedicated program with a
nice standard (or as good as) user interface - so I'll bore (
) a little bit longer.
Qt itself has a
add-a-plugin ability - see "The Lower-Level API: Extending Qt Applications". I am sure the developer/s know it and have already considered it in the past and have perfect reasons for not implementing it. I suppose one of the reasons is amount of work involved. What I would like to know are the other reasons, if any, which are perhaps obvious when one knows insides of Qt and Sigil - but not to outsider like me.