[Qbs] Branch 3.1 created

[Christian Kandeler]

Hi,

we have branched off Qbs 3.1. Scheduled release date for version 3.1.0 
is end of October (together with Qt Creator 18.0.0).

Contributors are requested to push bug fixes and small, zero-risk
improvements to this branch, while features continue to go to master.
Patches currently under review can be moved if they are reasonably close to
finished.


Christian