Provide a Project property to specify the minimum required QBS version

I really think we should have a requiredQbsVersion property on the Project item. If this property is defined and set to a value matching /^[0-9]+(\.[0-9]+){1,2}$/, QBS should issue an error when loading the project if the QBS version does not meet the requirement (except perhaps if -k or --force is in effect). If undefined, the project simply loads normally (possibly blowing up later).

We move so fast at this point and have fixed so many bugs and added so many new features that even for me as a QBS developer it's hard to keep track of whether the particular QBS builds I have installed in various places are reasonably capable of building my projects.

Other build systems and build-related tools have similar features - qmake, CMake, WiX, etc.

Obviously, the sooner to implement this (hopefully for the 1.2 target or even 1.1.1 if possible), the better, to avoid a chicken and egg problem.