Kill off UFO plugins.
Revert "Kill off UFO plugins."

This reverts commit 75cc7d3a4571089479a770f6efe2823e500b685f.
Remove UFO:AI plugin.
Add a little more context to configuration errors.
Add light_spot to the list of recognized light classes.
Merge branch 'master' of https://github.com/TTimo/GtkRadiant