Detect and warn of improper extension versions

The Guacamole web application should detect and warn the user of the presence of old, incompatible extensions, rather than simply blindly loading them and failing.

guacd may be able to detect this simply through comparing which version of libguac was linked, but as plugins for guacd are generally built at the same time as the rest of guacamole-server, this isn't as pressing a need.