[LyX/master] Prevent branch background color from leaking out of the document

[Jean-Marc Lasgouttes]

Le 21/01/2021 à 08:19, Jürgen Spitzmüller a écrit :
> BTW if you want to have fun, fire up LyX 2.3.x, define a new branch
> named "foreground" and see how all text (in all opened documents!)
> magically disappears.

Amusing indeed :)

This is why IMO declaring a name for such colors is a mistake. An enum 
value generated on the fly would be enough.

JMarc