Marking inverted branch insets

[Yuriy Skalko]

> Done in the attached patch. Now just with adding "~" to inverted insets, I think it is more visible than "!".
> 
> Yuriy

I've checked implementing the additional color change. But to retain LyX 
flexibility, just adding the "inverted branch label" color to the table 
and overriding labelColor method will be not enough. We'll also have to 
define a new property for layouts, something like "InactiveLabelFont". 
So probably it is not worth it. Also after usage of the patch for 
several days, I can say that adding only an inversion symbol works well 
for distinguishing inverted branch insets.


Yuriy