making qbs::InstallOptions::defaultInstallRoot() absolute

I want to display the default install root in creator.

qbs::InstallOptions::defaultInstallRoot() returns a path relative to the build directory.

The public qbs API does not seem to provide a way to find the build directory this is relative to, so could you please add a way to find the build directory (e.g. as a method on qbs::Project) or make the defaultInstallRoot() return an absolute path if possible?