Marking inverted branch insets

[Yuriy Skalko]

> I tried to do something a year ago, and my plan then was to strikeout the branch name. Unfortunately, it is not possible to do now, unless we can set he label as html (didn't try).
> 
> 
> JMarc


There is another way to get strikeout text -- Unicode. I tried it with 
some copied text from https://yaytext.com/strike/, but its rendering by 
Qt in LyX is not so good.

I agree that for most people (who don't know Boolean algebra) strikeout 
can be the most understandable indication for inversion.

So, is this patch OK to commit to have acceptable solution for now?


Yuriy