Add support for CSIDL_COMMON_APPDATA in QDesktopServices::storageLocation()

It is sometimes desireable to get access to a common data directory that does not depend on the current user.
(On Windows XP/Vista, SHGetFolderPath(NULL, CSIDL_COMMON_APPDATA, NULL, 0, szAppData) is the correct call for this.)

The closest thing I found was QDesktopServices::DataLocation, which returns a app-data dir in the current user's directory.