Update versioninfo

This contains 2 small new features for the build system:
- When in a git repository, use the output of "git describe" as version. Most notably, this affects the window title of Performous and the file version in windows builds.
- Update the code that creates the Windows file version to cope with git version numbers. I think my new code is also a bit easier and robuster ;)

PS: I bumped the "fallback" version to "1.0+" in the process...