[Qbs] Undocumented SubProject.condition property

[Jochen Ulrich]

Hi!

We found that there is an undocumented `condition` property on SubProject items.
It seems to have the same effect as setting the `condition` property inside the Properties block.
However, I find it more readable to have the condition on the SubProject instead of inside the Properties block.

So the question is: what's the state of this undocumented property?
Should it be made official and be documented or is it deprecated and will possibly be removed?

Best
Jochen