DEV-43439: Added calls to query the viewer version.

Rather than copy/paste the same version string formatting pattern
again, I added new string-based version routines:

/// return the full viewer version as a string like "2.0.0.200030"
const std::string &llGetViewerVersion();

/// return the viewer version as a string like "2.0.0"
const std::string &llGetViewerShortVersion();

/// return the viewer build version as a string, e.g., "200130"
const std::string &llGetViewerBuildVersion();