Adding build-set to working file set

[Alexander Dymo]

> Where did those icons come from? They are only created when you actually
> use them.
>
> I think using working-sets couldn't be easer, all you need is your left
> mouse button:
That's right. I'm not speaking about switching between them, but rather that I 
didn't understand when they appear and when they disappear.

> So where does the confusion come from? How can we make it easier to use?
I don't know yet. My usecase was one KDevelop instance staying for a long time 
opened. There I just hacked on KDevelop. 
Then I was opening one-two more kdevelops. I opened some files there, closed 
some files there.
Now after two weeks of such work I ended up with 25 working sets on the long-
staying instance :)