Add support for using Groups in Modules

Would this be possible to add? An example use case would be adding a Group in the (future) Xcode module whose file pattern is "~/Library/MobileDevice/Provisioning Profiles/provision". This would allow Qbs to enumerate the available provisioning profiles, read their data, and select the most appropriate one based on product information.

Reasoning: generally you don't want to explicitly specify the full path to a provisioning profile, or even its UUID (from which you could determine the full path), but instead specify a name or let automatic selection pick the best one (this is what Xcode does).

Currently using a hacky workaround in my WIP: https://codereview.qt-project.org/#/c/107882/3/share/qbs/modules/xcode/xcode.qbs