OSInfo: use full Windows version when version string is user-facing.

This adds a new method, OSInfo::getOSDisplayableVersion(),
which returns a version string that is suitable for displaying
to regular users.

Furthermore, Mumble's User Information dialog and Murmur have been
modified to use this new method when displaying OS version strings
to users.

On Windows, the new method returns the full product version of the
OS, such as "Windows 8 Pro" (as opposed to 6.2.9200, which is the
equivalent result of the getOSVersion() method).

There are places in Mumble where the original "simple" version
string is required for compatibility or aesthetic reasons, such
as statistics and Mumble's HTTP user agent. Those places keep
the older version (by keeping their calls to the original
getOSVersion() instead of switching to the "displayable" variant).

It might seem overkill to jump through hoops to be able to show the
full product version, but it can be helpful for, say, server admins
to have access to the full product version if they need to
troubleshoot issues with their users.