make "base" the default fallback for Properties blocks

In Propierties blocks the fallback value when all conditions evaluate to
false was "undefined". This behavior was unexpected for most users that
tried to change Properties blocks in derived items.
Now the fallback is "base".

Task-number: QBS-711
Change-Id: I174ab2cd2b916057559e5de086916fd56bd6e74f
Reviewed-by: Christian Kandeler <christian.kandeler@theqtcompany.com>