Renamed llviewerversion to llversioninfo, to avoid confusion with
llversionviewer in llcommon (llversion is already used by llwindow).

Created new LLVersionInfo class with the following methods:

        static S32 getMajor();
        static S32 getMinor();
        static S32 getPatch();
        static S32 getBuild();
        static const std::string &getVersion();
        static const std::string &getShortVersion();
        static const std::string &getChannel();

All viewer code has been updated to use this API. Viewer code no
longer directly includes llversionviewer.h from llcommon.