[QBS] Build graph QDataStream format (IMPORTANT!)

[Christian Kandeler]

Hi,

we only noticed today that we had completely forgotten to version our 
build graph files with respect to the version of the QDataStream format 
which is used to read and write it. This means that if you built your 
project with qbs compiled against Qt4 and then built it again with qbs 
compiled against Qt5 (or vice versa), qbs would crash.
We have now fixed the QDataStream format version at the one used by Qt 
4.8.0, which is the lowest Qt version which you can use to build qbs. 
While this solves the aforementioned problem, it also means that loading 
existing build graphs might now fail, most likely with a crash. 
Therefore, when using qbs after patch 9df9b0e8, you must manually delete 
all existing build graphs if they were written by an older qbs built 
against Qt > 4.8.0.

Sorry for the inconvenience,
Christian